CATALYTIC    ACTION 


BY 

K.    GEORGE    FALK 

HARBIMAN  RESEARCH  LABORATORY 

THE  ROOSEVELT   HOSPITAL 

NEW  YORK 

Author  of  "The  Chemistry  of  Enzyme  Actions,' 
"Chemical  Reactions'' 


BOOK  DEPARTMENT 
The  CHEMICAL  CATALOG  COMPANY,  Inc. 

ONE  MADISON  AVENUE,  NEW  YORK,  U.  S.  A. 
1922 


F3 


COPYRIGHT,  1922,  BY 

The  CHEMICAL  CATALOG  COMPANY,  Inc. 
All  Rights  Reserved 


Press  of 

J.  J.  Little  &  Ives  Company 
New  York,  U.  S.  A. 


CONTENTS 

CHAPTER  PAGE 

I  INTRODUCTION;  GENERAL  VIEWS  ON  CATALYSIS       .         9 

II  REACTION  VELOCITY  AND  CATALYSIS       ...       23 

III  THEORY  OF  CATALYTIC  ACTIONS     ....       38 

IV  ENERGY  RELATIONS       .         .         .         .         .         .56 

V  RECENT  THEORIES  OF  CHEMICAL  ACTION       .         .       70 

VI    ENZYME  ACTIONS         .         .         .         .         .         .94 

VII    A  CHEMICAL  INTERPRETATION  OF  LIFE  PROCESSES  .     123 
VIII    CONTACT  CATALYSIS      .         «         .         .         .         .     140 

INDEX  166 


536:if>2 


PREFACE 

In  order  to  treat  the  phenomena  of  Catalytic  Actions  in  a 
systematic  manner,  it  is  necessary  to  define  or  describe  in  some 
way  the  relations  which  are  to  be  included.  At  the  same  time 
such  a  definition  or  description  must  recognize  a  definite  con- 
nection and  interrelation  with  chemical  reactions  in  general. 
Fundamentally,  the  same  general  laws  and  regularities  must  hold 
for  all  such  reactions,  but  Catalytic  Actions  are  distinguished  by 
certain  peculiarities  which  have  made  it  advisable  to  consider 
and  treat  them  as  a  separate  group. 

It  is  the  purpose  of  this  book  to  emphasize  the  chemical  nature 
of  so-called  Catalytic  Actions.  The  general  theory  of  the 
Mechanism  of  Chemical-  Reactions  elaborated  to  some  extent 
elsewhere  serves  as  the  groundwork.  The  reactions  which  are, 
by  more  or  less  common  consent,  included  under  Catalytic 
Actions,  are  then  determined  by  the  introduction  of  a  simple 
concept. 

Such  a  development  is  simple  in  principle.  At  the  same  time, 
the  expression  of  certain  doubts  may  be  permitted  in  this  con- 
nection. The  various  attempts  which  have  been  made  in  the 
past  to  define  the  terms  Catalysis  and  Catalytic  Action  have 
been  unsatisfactory  in  that  the  limitations  imposed  upon  the 
chemical  reactions  and  the  changes  involved,  have  been  inade- 
quate in  one  way  or  another.  The  writer  naturally  believes  that 
the  definition  which  he  suggests  for  Catalytic  Action  is  more 
satisfactory  than  any  heretofore  proposed,  but  the  uncertainties 
of  this  definition  are  also  pointed  out  in  the  pages  following. 
In  fact,  it  may  be  asked  whether  the  separation  of  Catalytic 
Actions  from  Chemical  Reactions  in  general  is  not  an  artificial 
one,  and  incapable  of  exact  formulation.  This  question  is  dis- 
cussed in  some  detail.  The  main  point,  however,  cannot  be 
emphasized  too  frequently;  namely,  that  chemical  (and  also 
physical)  relationships  and  laws  which  apply  to  Chemical  Re- 
actions, necessarily  must  hold  for  Catalytic  Actions,  no  matter 
how  these  may  be  limited  or  defined. 

7 


8  Preface 

No  attempt  has  been  made  to  present  all  the  reactions  or  even 
a  large  number  of  the  reactions  which  might  be  included  in  the 
phenomena  under  discussion.  A  more  or  less  definite  viewpoint 
is  presented  with  sufficient  examples  to  illustrate  the  general 
principles. 

The  writer  wishes  to  thank  those  who  have  aided  him  in  one 
way  or  another  to  develop  his  views  and  make  possible  their 
presentation.  The  manuscript  was  gone  over,  either  wholly  or 
in  part,  by  Professor  Jacques  Loeb  and  Dr.  J.  H.  Northrop  of 
the  Rockefeller  Institute  for  Medical  Research,  and  by  Professor 
George  B.  Pegram  of  Columbia  University,  and  some  changes 
were  incorporated  as  a  result  of  their  suggestions.  The  various 
relations  were  clarified  and  elaborated  greatly  in  consequence  of 
discussions  with  Professors  J.  M.  Nelson,  Ralph  H.  McKee,  and 
H.  T.  Beans  of  Columbia  University.  Finally,  the  writer  wishes 
to  express  his  indebtedness  to  Miss  Grace  McGuire  and  Miss 
Helen  Miller  Noyes,  his  associates  at  the  Harriman  Research 
Laboratory,  for  their  constant  co-operation  and  invaluable  aid 
in  the  writing  of  this  book  as  well  as  in  its  preparation  for  pub- 
lication. 

March  1,  1922. 


CATALYTIC  ACTION 

/ 

Chapter  I. 
Introduction :    General  Views  on  Catalysis. 

The  science  of  Chemistry  may  be  said  to  deal  with  (1)  the 
properties  and  compositions  of  substances;  (2)  changes  in  these 
properties  and  compositions  which  accompany  interactions  be- 
tween various  substances;  and  (3)  changes  which  take  place 
under  the  influence  of  changes  in  external  conditions.  The  ex- 
pression of  the  laws  and  regularities  which  are  involved  in  such 
properties,  compositions,  and  changes,  forms  the  main  problem 
of  chemistry. 

The  so-called  physical  properties  of  substances  and  mixtures, 
such  as  crystal  form,  fusion  temperature  (or  melting  point), 
vapor  pressure,  specific  gravity  (or  density),  color  (light  absorp- 
tion and  reflection),  heat  conductance,  electric  conductance,  etc., 
may  be  determined  by  suitable  measurements  directly  on  the 
substances  themselves.  In  order  to  determine  the  chemical  com- 
positions of  substances,  or  to  find  their  relations  to  other  sub- 
stances in  the  sense  that  certain  reactions  or  phenomena  are 
common  to  both  and  therefore  indicate  a  common  constituent, 
it  is  necessary  to  submit  the  substances  in  question  to  treatments 
which  cause  changes  in  them.  Such  treatments  and  the  changes 
in  composition,  and  consequently  properties,  of  certain  constitu- 
ents are  included  under  the  general  term  "chemical  reactions." 
It  is,  therefore,  fairly  obvious  that  the  study  of  chemical  reactions 
is  one  of  the  most  important  of  the  problems  which  can  be  taken 
up  in  connection  with  the  science  of  chemistry.  The  determina- 
tion of  the  compositions  of  substances  depends  upon  chemical 
reactions,  the  estimation  of  various  constituents,  the  formation 
of  various  products,  and  in  fact  practically  every  change  which 

9 


10  CATALYTIC  ACTION 

is  reflected  in  a  change  in  properties  involves  in  one  form  or 
another  a  chemical  reaction.  Chemical  reaction  always  involves 
change.  It  is  dynamic  in  nature  and  is  as  a  rule  of  greater 
complexity,  and  perhaps,  therefore,  of  more  profound  interest, 
than  the  static  side  of  chemistry. 

Recently  the  writer  published  a  treatise  on  "Chemical  Reac- 
tions; Their  Theory  and  Mechanism,"  *  in  which  a  general  theory 
of  chemical  reactions  was  developed,  based  primarily  upon  the 
"addition  compound  formation"  view.  A  number  of  the  more 
modern  concepts  of  chemistry  were  used  and  it  was  shown  that 
this  theory  or  viewpoint  permitted  the  inclusion  of  all  chemical 
reactions,  those  classed  heretofore  as  organic  as  well  as  those 
included  under  the  group  inorganic.  Various  new  relations  were 
pointed  out  as  following  from  this  method  of  treatment  of  chemi- 
cal reactions. 

Since  the  importance  of  chemical  reactions  is  so  great  in 
chemical  practise  and  theory,  it  is  of  interest  to  follow  some  of 
the  lines  which  have  been  developed  in  this  field  in  the  course  of 
the  past  hundred  years.  The  present  book  is  an  attempt  to 
present  a  group  of  reactions  which  has  been  separated  from  the 
group  of  chemical  reactions  in  general  and  treated  as  a  special 
class  showing  certain  distinctive  relations.  At  the  same  time, 
the  more  general  points  of  view  applicable  to  all  chemical  reac- 
tions will  be  retained  here  and  emphasized  wherever  possible. 

Catalytic  actions  have  been  grouped  for  a  number  of  years  as 
a  distinct  class  of  chemical  reactions  having  certain  properties 
or  relations  in  common.  The  importance  which  has  been  ascribed 
to  them  is  evident  from  the  number  of  publications,  both  in  the 
form  of  books  and  of  articles,  which  have  appeared  and  are  still 
appearing.  They  will  not  be  taken  up  here  from  the  point  of 
view  of  their  own  distinctive  behavior  primarily,  but  rather,  their 
relations  to  chemical  reactions  in  general  will  be  described  and 
the  points  discussed  in  which  they  are  supposed  to  differ  from 
these.  In  this  way  it  is  hoped  to  show  the  relations  of  catalytic 
reactions  to  other  reactions  and  in  a  measure  to  unify  the  phe- 
nomena of  chemistry  in  place  of  creating  new  branches  or  sub- 
divisions. Also,  it  will  be  seen  that  catalytic  reactions  exemplify 
certain  relations  which  are  not  as  apparent  in  other  reactions. 

1  The  D.  Van  Nostrand  Company,  New  York,  1920. 


INTRODUCTION:    GENERAL  VIEWS  ON  CATALYSIS  11 

In  this  way  a  study  of  such  reactions  may  extend  and  strengthen 
the  concepts  of  chemical  theory. 

In  studying  a  certain  group  of  reactions  such  as  catalytic 
reactions,  which  are  separated  from  reactions  in  general  because 
of  possessing  certain  properties  or  characteristics  or  showing 
certain  behaviors,  it  is  first  of  all  necessary  to  state  what  these 
properties  or  characteristics  or  behaviors  may  be,  or  in  other 
words,  to  define  as  accurately  as  is  possible  the  meaning  of  the 
term  catalytic  reaction  or  catalysis.  An  accurate  definition 
would  simplify  greatly  the  study  of  the  problem,  while  a  defini- 
tion or  description  which  speaks  in  more  or  less  general  terms  of 
the  phenomena  in  question  will  lead  to  greater  or  less  uncer- 
tainties in  the  treatment.  In  the  following  considerations,  it  will 
be  pointed  out  that  the  definitions  which  are  generally  given  for 
catalysis  and  catalytic  actions  have  developed  in  a  gradual 
manner;  that  the  phenomena  which  have  been  included  were 
frequently  not  well  characterized,  and  in  fact  often  not  charac- 
terized at  all;  that  a  somewhat  top-heavy  theoretical  structure 
has  been  developed  in  which  an  increased  knowledge  of  the 
mechanism  of  certain  reactions  did  not  aid  materially  in  the 
general  views  on  catalysis;  and  that  finally,  the  consideration  of 
catalytic  reactions  as  compared  with  other  reactions,  may  be 
shown  to  lead  to  a  simple  definition  which  appears  to  be  of  gen- 
eral applicability. 

The  conception  of  catalysis  was  introduced  into  Chemistry 
in  1836  by  J.  Berzelius  in  order  to  group  together  a  number  of 
apparently  diverse  chemical  reactions  and  to  indicate  that  the 
mechanism  of  these  reactions  might  be  ascribed  to  a  common 
cause.  The  reactions  which  he  considered  included  the  change 
of  starch  into  sugar  in  the  presence  of  dilute  acids,  the  acid 
remaining  unchanged; *  the  decomposition  of  hydrogen  peroxide 
with  evolution  of  oxygen  in  alkaline  solution  in  the  presence  of 
manganese,  silver,  platinum,  gold,  or  fibrin;2  the  oxidation  of 
ethyl  alcohol  to  acetic  acid  by  finely  divided  platinum; 3  the 
spontaneous  combination  of  hydrogen  and  oxygen  in  the  pres- 
ence of  cold  spongy  platinum ; 4  the  same  reaction  in  the  pres- 

13.  Kirchhof,  ScJiweigger's  Journ.  4,  108   (1812). 

2  J.  Thgnard,  Ann.  Chim.  Phya.  9,  314  (1818). 

«E.  Davy,  Phil.  Trans.  100,  108  (1820). 

*  J.  W.  Dobereiner,  Schweigger's'  Journ.  31,,  91   (1822)  ;  38,  321   (1823). 


12  CATALYTIC  ACTION 

ence  of  heated  gold,  silver,  or  glass;1  and  finally,  the  conversion 
of  alcohol  into  ether  in  the  presence  of  sulfuric  acid.2 

The  views  of  Berzelius,  based  upon  the  study  of  these  reac- 
tions may  be  best  given  in  his  own  words. 

"It  is  then  proved  that  several  simple  and  compound  bodies,  soluble 
and  insoluble,  have  the  property  of  exercising  on  other  bodies  an  action 
very  different  from  chemical  affinity.  By  means  of  this  action  they  pro- 
duce, in  these  bodies,  decompositions  of  their  elements  and  different 
recombinations  of  these  same  elements  to  which  they  themselves  remain 
indifferent. 

'This  new  force,  which  was  hitherto  unknown,  is  common  to  organic 
and  inorganic  nature.  I  do  not  believe  that  it  is  a  force  quite  independent 
of  electrochemical  affinities  of  matter;  I  believe,  on  the  contrary,  that  it  is 
only  a  new  manifestation  of  the  same;  but,  since  we  cannot  see  their  con- 
nection and  mutual  dependence,  it  will  be  more  convenient  to  designate  the 
force  by  a  separate  name.  I  will  therefore  call  this  force  the  catalytic  jorce 
and  I  will  call  catalysis  the  decomposition  of  bodies  by  this  force  in  the 
same  way  that  one  calls  by  the  name  analysis  the  decomposition  of  bodies 
by  chemical  affinity. 

"The  body,  which  brings  about  the  changes  in  the  constituents,  does 
so  not  by  itself  taking  part  in  the  new  compounds;  it  remains  unchanged 
and  acts  therefore  by  means  of  an  internal  force,  whose  nature  is  still 
unknown  to  us,  at  the  same  time  that  its  existence  is  made  known  in  this 
way.8 

"Certain  bodies  exert  by  contact  with  other  bodies  an  influence  such 
that  a  chemical  effect  ensues,  compounds  are  destroyed  or  formed  afresh, 
without  the  substances,  whose  presence  brings  this  about,  taking  part  to 
the  slightest  degree. 

"This  force  is  exerted  more  generally,  but  more  mysteriously,  in  the 
processes  of  organic  chemistry,  especially  in  living  bodies;  generally  there 
appears  to  be  no  other  probable  reason  for  the  manifold  products  which 
are  formed  in  the  living  plant  or  animal  from  one  and  the  same  common 
fluid  which  is  brought  in  contact  with  the  different  parts,  except  that  the 
solid  parts  cause  different  changes  in  different  places  in  the  constituents 
of  the  added  fluids.  .  .  ." 4 

In  considering  the  views  of  Berzelius  it  must  be  remembered 
that  the  concepts  of  "force"  and  of  "chemical  affinity"  were  not 
as  clear  and  well  defined  at  the  time  he  developed  the  classifica- 
tion of  catalytic  actions  as  they  are  at  the  present  time.  The 
importance  which  must  be  ascribed  at  the  present  time  to  Ber- 
zelius' definition  and  description  lies  in  the  fact  that  the  criteria 
which  he  used  to  determine  whether  a  reaction  is  to  be  classed  as 
catalytic  are  essentially  those  which  the  majority  of  chemists 
still  use  for  the  same  purpose.  Only  if  catalytic  reactions  are 
considered  as  a  group,  are  more  general  and  more  exact  relations 

1  P.  L.  Dulong  and  J.  Th6nard,  Ann.  Chim.  Phys.  (ii)  23,  440;  24,  380 
(1823) . 

2E.  Mitcherlich,  Fogg.  Ann.  31,  273   (1834). 

tJahresber.  d.  Chem.  1836,  237. 

4  Lehrbuch  der  Chemie,  V.  Edition,  1848,  Vol.  1,  pp.  111-2 


INTRODUCTION:    GENERAL  VIEWS  ON  CATALYSIS  13 

used.  For  the  general  descriptive  relations,  most  chemists  have 
not  gone  beyond  the  views  of  Berzelius.  In  studying  the  reac- 
tions given  and  the  statements  of  Berzelius  it  is  evident  that 
the  reason  for  this  lies  in  the  fact  that  the  common  cause  for 
separating  catalytic  reactions  from  chemical  reactions  in  general 
is  taken  to  be  the  confession  of  ignorance  of  the  nature  and 
mechanism  of  the  reaction  in  question. 

This  conservatism  of  many  chemists  does  not  mean  however 
that  advances  have  not  been  made  in  the  classification  of  cata- 
lytic actions,  the  theoretical  concepts  involved,  and  the  relations 
to  other  chemical  and  physical  theories.  Striking  advances  have 
been  made,  especially  by  Ostwald,  Bredig,  van't  Hoff,  and  others, 
but  these  advances  are  very  frequently  ignored  in  the  general 
chemical  literature.  If  this  is  the  case,  it  would  appear  that 
the  advances  were  not  fundamentally  sound,  or  if  sound,  not 
useful.  Although  this  may  not  be  the  best  place  to  discuss  the 
question,  and  also  perhaps  anticipates  the  later  discussions 
of  catalytic  reactions,  it  may  be  asked  whether  this  classification 
of  reactions  based  upon  an  unknown  factor  has  served  a  useful 
purpose.  The  answer  to  this  question  must  be  an  unqualified 
affirmative.  It  has  been  possible  to  group  a  number  of  appar- 
ently unrelated  reactions  and  has  focussed  attention  upon  the 
importance  of  these  reactions.  Whether  or  not  this  classification 
is  of  permanent  use  and  value  is  of  secondary  significance.  That 
further  developments  resulted  in  calling  attention  to  the  illusory 
nature  of  the  unknown  general  relationship,  that  the  term  catal- 
ysis for  a  time  became  in  a  sense  a  term  of  disrepute,  signifying 
ignorance  in  place  of  knowledge  of  the  nature  of  the  reaction 
associated  with  it,  is  of  interest  in  the  historical  development. 
Using  catalysis  to  call  attention  to  unknown  factors  in  chemical 
reactions  often  temporarily  relieved  the  minds  of  those  applying 
the  term,  but  the  real  result  has  been  to  call  attention  to  the 
puzzling  nature  of  the  problem  involving  the  mechanism  of  the 
given  reaction.  If  after  reviewing  some  of  the  different  aspects 
of  the  problem,  it  should  appear  to  some  that  it  would  be  advis- 
able no  longer  to  use  the  term  catalysis,  this  should  not  be  taken 
in  any  way  as  belittling  the  great  service  which  the  concept  has 
rendered  in  the  past,  or  the  truly  remarkable  insight  of  Berzelius 
and  others  in  developing  catalytic  relations  in  various  directions. 


14  CATALYTIC  ACTION 

The  subject  of  catalysis  may  be  studied  in  several  different 
ways.  One  of  the  most  interesting  would  be  to  adopt  the  defini- 
tions and  descriptions  given  by  Berzelius  and  describe  the  state 
of  chemical  knowledge  and  theory  of  his  time.  Then  the  views 
of  the  present  time  might  be  given,  and  the  significance  of  catal- 
ysis as  at  present  understood.  Such  a  presentation  would  in- 
volve giving  the  theoretical  conceptions  of  chemistry  of  the  two 
periods  as  they  bear  upon  catalysis.  These  two  cross-sections  of 
chemical  knowledge  and  theory  centered  upon  catalysis  would 
necessarily  develop  into  a  historical  comparison  of  the  chemical 
knowledge  of  the  periods  of  1840  and  of  1920  as  related  to  catal- 
ysis. This  would  furnish  interesting  matter  for  discussion  but 
it  would  hardly  be  profitable  in  the  present  connection  to  enter 
minutely  into  such  a  comparison.  The  danger  would  lie  in  giv- 
ing a  one-sided  historical  view  of  the  two  periods  in  such  a 
presentation,  since  necessarily  many  problems  and  theories  might 
have  been  developed  which  apparently  were  not  related  at  the 
time  to  the  problem  in  question  and  would  therefore  have  been 
omitted.  Also,  the  views  held  at  present  by  different  workers  do 
not  agree  in  all  particulars,  and  the  presentation  would  consist 
of  a  comparison  of  a  fairly  accurate  historical  view  of  the  older 
period  with  a  more  or  less  personal  view  of  the  present  period. 

A  second  method  of  treating  catalysis  would  be  the  en- 
cyclopedic method  which  involves  a  complete  and  detailed 
account  of  catalytic  reactions  including  the  historical  develop- 
ment. Such  treatments  have  been  given  at  different  times,  either 
covering  the  greater  part  of  the  field  of  reactions  and  theories 
which  have  been  taken  up  in  connection  with  catalysis,  or  deal- 
ing with  more  or  less  specialized  subdivisions  of  the  field  which 
have  been  suggested  at  various  times.  It  would  be  invidious  to 
single  out  any  special  treatment  for  reference. 

Since  this  field  has  been  covered  in  a  satisfactory  manner, 
there  does  not  appear  to  be  any  reason  at  present  to  add  another 
compilation  to  the  list. 

Another  way  of  treating  the  subject  of  catalysis  is  to  take 
up  separate  topics,  each  including  a  number  of  related  or  similar 
reactions,  and  discussing  the  relations  separately.  Thus,  the  fol- 
lowing topics  might  be  chosen  to  describe  the  general  phenomena 
of  catalysis. 


INTRODUCTION:    GENERAL  VIEWS  ON  CATALYSIS  15 

Catalytic  actions  connected  with  hydrogen  ions. 
Catalytic  actions  connected  with  hydroxyl  ions. 
Catalytic  oxidation-reduction  actions. 
Catalysis  in  homogeneous  solutions. 
Catalysis  in  heterogeneous  systems. 
Enzyme  actions. 
Etc.,  etc. 

Such  a  presentation  would  include  a  somewhat  detailed  dis- 
cussion of  reactions  more  or  less  typical  for  each  group  as  well 
as  the  general  characteristics  of  the  different  groups.  Many 
valuable  points  might  be  obtained  in  this  way,  but  the  weakness 
of  this  method  of  presentation  lies  in  the  fact  that  after  charac- 
terizing the  predominating  features  of  each  group,  when  com- 
paring the  different  groups  with  each  other,  attention  is  likely 
to  be  drawn  to  the  differences  shown  by  the  various  subdivisions. 
Since  attempts  at  advance  in  the  scientific  treatment  of  chemical 
phenomena  appear  to  be  more  useful  if  similarities  are  brought 
out  and  apparently  diverse  phenomena  correlated,  dividing  the 
field  of  catalytic  actions  into  a  number  of  smaller  groups,  while 
useful  in  some  ways,  will  only  be  adopted  secondarily  in  the 
present  instance. 

From  what  has  been  said  so  far,  it  may  be  seen  that  the  object 
here  is  to  define  as  accurately  as  is  possible  the  group  of  catalytic 
reactions;  to  show  the  relations  of  these  catalytic  reactions  to 
chemical  reactions  not  included  under  catalytic;  and  to  find,  if 
possible,  general  relations  underlying  the  cause  or  mechanism 
or  whatever  it  may  be  called,  of  these  reactions. 

The  method  of  treatment  to  be  adopted  appears  to  reverse 
the  traditional,  time  honored  method  of  considering  chemical 
phenomena  and  theories.  In  place  of  describing  a  large  amount 
of  data  and  facts  and  developing  hypotheses  and  theories  from 
these,  general  viewpoints  will  be  adopted  wherever  possible,  and 
relations  to  other  branches  of  science  will  be  indicated.  At  the 
same  time  as  many  illustrations  as  practicable  will  be  given. 
This  is  not  an  elementary  treatise  of  catalytic  actions.  The 
reader  is  assumed  to  be  sufficiently  familiar  with  general  chemi- 
cal reactions  to  fill  out  the  necessary  details. 

In  this  chapter,  the  general  status  of  the  views  held  with 
regard  to  catalytic  actions  is  being  presented.  The  development 
of  these  views  is  marked  by  two  outstanding  conceptions.  The 
first  was  that  of  Berzelius  in  which  the  term  catalysis  was  pro- 


16  CATALYTIC  ACTION 

posed  to  include  the  phenomena  in  which  a  chemical  reaction 
takes  place,  in  some  way  because  of  the  presence  of  a  substance 
apparently  not  involved  in  the  reaction.  The  second  great  theo- 
retical advance  was  due  to  W.  Ostwald  who  in  1894  and  the 
succeeding  years  developed  the  significance  of  reaction  velocity 
in  catalytic  phenomena.  "Catalysis  is  the  acceleration  of  a 
slowly  proceeding  reaction  by  the  presence  of  a  foreign  body. 
.  .  .  There  are  numberless  substances  or  combinations  of  sub- 
stances, which  are  in  themselves  not  stable,  but  in  a  state  of 
slow  transformation  and  therefore  appear  to  be  stable,  because 
their  transformations  take  place  so  slowly  that  they  are  not 
appreciable  in  the  short  time  of  observation  generally  employed. 
Such  substances  or  systems  frequently  attain  an  accelerated  rate 
of  transformation,  if  certain  foreign  substances,  i.e.,  substances 
not  necessary  for  the  reaction  itself,  are  present.  This  accelera- 
tion occurs  without  change  in  the  general  energy  relationships, 
since  after  the  reaction  is  completed,  the  foreign  substance  can 
be  considered  to  be  removed  from  the  reaction  sphere,  so  that 
the  energy  which  may  have  been  used  up  in  adding  it  is  regained 
by  its  removal,  or  the  reverse.  The  changes  must,  however,  as 
in  all  natural  transformations,  occur  in  the  sense  that  the  free 
energy  of  the  whole  system  decreases.  It  is  misleading,  how- 
ever, to  look  upon  catalytic  action  as  a  force,  which  brings  about 
something  which  would  not  take  place  in  the  absence  of  the 
catalytically  active  substance."  * 

The  typical  examples  of  catalytic  actions  which  may  be  men- 
tioned in  this  connection  include  the  hydrolysis  of  esters,  of 
sucrose,  and  of  acetamide  by  acids,  different  acids  producing 
different  changes  in  velocities,  but  the  acid  producing  each  effect 
being  unchanged  at  the  end  of  the  reaction.  A  great  part  of 
the  experimental  data  bearing  upon  these  reactions  was  due  to 
Ostwald.2  It  remained  for  Arrhenius  to  show  that  these  cata- 
lytic actions  of  acids  might  better  be  referred  to  the  actions  of 
the  hydrogen  ions  formed  by  the  electrolytic  dissociation  of  the 
acids.  This  last  was  a  great  step  forward  in  the  correlation  of 
the  experimental  data,  and  it  has  only  been  in  recent  years  that 
the  view  of  the  catalytic  actions  of  hydrogen  (and  also  of 

JW.  Ostwald;  review  of  an  article  by  F.  Stohmann,  Z.  phvsiJc.  Ghent.  15 
705  (1894). 

2W.  Ostwald,  J.  pr.  Chem.  27,  1;  28,  449  (1883)  ;  29,  385;  SO,  93  (1884). 


INTRODUCTION:    GENERAL  VIEWS  ON  CATALYSIS  17 

hydroxyl)  ions  was  shown  to  be  incomplete  and  to  necessitate 
modification.  This  question  will  be  taken  up  again  in  a  later 
chapter.  It  may  also  be  pointed  out  that  the  experimental 
studies  of  G.  Bredig  and  his  co-workers  added  considerably  to 
the  quantitative  knowledge  of  various  catalytic  reactions. 

Berzelius  had  given  a  qualitative  meaning  to  the  term  catal- 
ysis; Ostwald  gave  it  a  quantitative  meaning.  Following  Ost- 
wald, it  was  possible  to  give  quantitative  measurements  in  con- 
nection with  catalytic  phenomena  and  to  fix  the  significance  of 
the  terms  more  satisfactorily.  This  advance  was  only  possible 
because  of  the  advance  in  chemical  theory  in  connection  with 
chemical  affinity  and  its  measurement,  and  because  of  the  experi- 
mental and  theoretical  studies  of  reaction  velocities  or  chemical 
kinetics  which  had  systematized  the  relations  and  had  success- 
fully applied  mathematical  equations  to  chemical  equations  in 
so  far  as  the  rates  of  change  of  the  reactions  represented  by  these 
equations  were  concerned.  The  views  brought  out  and  elabo- 
rated by  Ostwald  in  various  publications  accepted  the  proposi- 
tion of  Berzelius  that  a  chemical  reaction  took  place  in  the  pres- 
ence of  a  foreign  body  known  as  the  catalyst.  He  added  thereto 
the  proposition  that  the  velocity  of  a  chemical  reaction  was 
changed  by  the  presence  of  the  foreign  body,  foreign  in  the  sense 
that  it  apparently  did  not  take  part  in  the  reaction,  or  in  other 
words,  that  it  appeared  at  the  end  of  the  reaction  apparently 
unchanged  as  compared  with  its  condition  at  the  beginning  of  the 
reaction. 

It  is  of  interest  historically  in  connection  with  the  advance 
made  by  Ostwald  in  the  theoretical  treatment  of  catalytic  actions 
to  note  that  Wilhelmy  *  in  his  study  of  the  action  of  acids  on 
sucrose  in  which  the  law  of  mass  action  was  formulated  and 
applied  quantitatively  for  the  first  time,  stated  "I  must  leave  it 
to  the  chemists  to  decide,  whether  and  how  far  the  formulas 
obtained  are  applicable  to  other  chemical  processes;  in  any  case, 
all  those  processes  to  which  one  ascribes  the  operation  of  a  cata- 
lytic force  seem  to  me  to  belong  to  this  class." 

The  terms  "catalytic  action,"  "catalytic  forces,"  "catalysts" 
or  "catalyzers,"  following  the  classification  of  Berzelius  and  as 
modified  in  a  number  of  minor  particulars  by  other  workers  in 

»L.  Wilhelmy,  Pogg.  Ann.  81,  413,  499   (1850). 


18  CATALYTIC  ACTION 

this  field,  were  used  in  a  more  or  less  indeterminate  way  to 
include  a  number  of  chemical  reactions  which  were  influenced 
in  an  unknown  manner  by  substances  present  but  apparently  not 
involved  in  the  reactions.  For  a  number  of  years,  and  in  fact 
frequently  at  the  present  time,  the  advances  in  chemical  theory 
were  not  used  in  connection  with  catalytic  reactions  or  in  denot- 
ing a  reaction  as  catalytic.  As  a  natural  result,  these  reactions 
and  the  term  catalytic  actions  became  known  as  a  sort  of  gen- 
eral group  of  reactions  to  which  any  unknown  factors  in  the 
mechanism  of  chemical  reactions  were  ascribed.  The  scientific 
knowledge  of  such  reactions  was  therefore  not  advanced,  and  in 
fact  in  a  number  of  cases  it  was  retarded,  by  the  camouflaging  of 
the  ignorance  represented  by  the  term  "catalytic."  Thus,  in  the 
recent  monograph  of  S.  C.  Lind,1  one  of  the  reasons  for  the  slow 
development  of  Photochemistry  was  stated  to  have  been  "the 
early  unfortunate  over-emphasis  of  the  catalytic  nature  of  photo- 
chemical phenomena." 

The  reason  for  this  unfortunate  development  in  catalysis 
cannot  be  ascribed  to  the  classification  outlined  by  Berzelius, 
who  advanced  chemical  theory  in  this  particular  as  in  many 
others,  but  rather  to  the  blind  following  of  those  who  used  his 
views  without  considering  the  advances  which  had  taken  place 
both  in  the  more  extended  knowledge  of  the  mechanism  of  reac- 
tions and  in  the  theoretical  viewpoints  which  had  been  developed. 
As  stated,  it  remained  for  Ostwald  to  put  the  views  on  catalysis 
on  a  more  definite  and  satisfactory  basis  by  pointing  out  the 
significance  of  reaction  velocities.  The  more  detailed  discussion 
of  reaction  velocities  in  relation  to  catalysis  will  be  presented 
in  the  following  chapter. 

As  a  result  of  the  large  amount  of  experimental  work  which 
was  accumulated  under  the  general  heading  of  catalysis,  certain 
criteria  were  developed  which  were  considered  necessary  for  a 
reaction  to  be  placed  definitely  in  the  indicated  category.  As  it 
is  desired  to  present  in  this  chapter  an  outline  of  the  views  on 
catalysis  which  are  held  more  or  less  commonly  at  present,  these 
criteria  will  now  be  stated  as  given  in  one  of  the  most  satis- 

1 S.  C.  Lind,  "The  Chemical  Effects  of  Alpha  Particles  and  Electrons" ; 
American  Chemical  Society,  Monograph  Series.  The  Chemical  Catalog  Co,, 
Inc.,  New  York,  1921,  Page  19. 


INTRODUCTION:    GENERAL  VIEWS  ON  CATALYSIS  19 

factory  of  the  recent  books  on  the  subject.1    Omitting  the  de- 
tailed discussion  given  by  the  authors,  they  are  as  follows: 

"(1)  The  chemical  composition  of  the  catalytic  agents  is  unchanged  on 
completion  of  the  reaction  process. 

"(2)  Minimal  amounts  of  a  catalytic  agent  are  adequate  for  the  trans- 
formation of  large  quantities  of  the  reacting  substances. 

"(3)  A  catalyst  cannot  affect  the  final  state  of  equilibrium. 

"(4)  A  catalyst  modifies  the  velocity  of  two  inverse  reactions  to  the 
same  degree. 

"(5)  A  catalytic  agent  is  incapable  of  starting  a  reaction;  it  can  only 
modify  the  velocity  of  the  reaction." 

It  is  shown  that  on  the  basis  of  thermodynamics,  criteria  (3) 
and  (4)  follow  from  (1)  and  (2) ;  that  as  a  corollary  from  (3) 
and  (4),  "it  follows  of  necessity  that  the  state  of  equilibrium 
is  independent  of  the  nature  and  quantity  of  the  catalytic 
agent." 

In  the  later  chapters  of  this  book  an  attempt  will  be  made 
to  show  the  applications  and  some  of  the  possible  limitations 
of  these  criteria.  The  topics  will  not  be  taken  up  consecutively, 
but  the  general  discussion  may  serve  to  show  the  relations  with- 
out taking  up  each  point  separately.  At  the  same  time,  while 
these  criteria  are  given  as  the  result  of  the  careful  study  of  a 
large  number  of  reactions  and  have  been  accepted  in  general 
terms  for  a  period  of  years  (fifteen  years  or  more)  a  review  of 
the  articles  on  catalytic  reactions  even  of  recent  years  does  not 
indicate  that  the  criteria  have  been  taken  into  account  in  the 
classification  and  consideration  of  many  of  these  reactions. 
Only  in  isolated  cases,  has  the  treatment  been  guided  by  the 
various  criteria.  The  classification  generally  used  is  essentially 
that  of  Berzelius  -with  the  addition  of  the  change  in  velocity  as 
postulated  by  Ostwald.  This  cannot  be  due  alone  to  the  failure 
of  chemists  to  adopt  the  theoretical  developments,  but  may  be 
more  probably  ascribed  either  to  the  inadequacy  of  the  theo- 
retical views,  or  to  the  indefiniteness  of  the  definitions,  or  to  the 
multiplicity  of  criteria  or  requirements  involved  in  the  classifica- 
tions. This  question  will  be  developed  further  in  later  chapters. 

In  various  treatments  and  compilations  of  catalytic  actions, 
attempts  have  not  been  lacking  to  present  classifications  of  cata- 
lytic reactions  and  reagents  in  order  to  systematize  the  relations. 

1 E.  K.  Rideal  and  H.  S.  Taylor ;  "Catalysis  in  Theory  and  Practice," 
MacmiJlan  $  Co.,  Ltd.,  London,  1919.  Chapter  I,  on  "Criteria  of  Catalysts," 


20  CATALYTIC  ACTION 

None  of  these  systems  of  classification  has  been  found  entirely 
satisfactory.  In  order  to  show  their  nature,  that  developed  by 
Ostwald  *  will  be  outlined: 

I.   Crystallization  from  supersaturated  solutions.    E.g.,  the  crystallization 
•    of  sodium  sulfate  from  a  supersaturated  solution  in  the  presence  of 

a  flake  of  dust  or  the  fragment  of  a  crystal. 

II.    Catalyses  in  homogeneous  systems.     E.g.,  the  action  of  acids  upon 
aqueous  solutions  of  cane  sugar. 

III.  Catalyses  in  heterogeneous   systems.    E.g.,   the   action   of   platinum 

upon  a  mixture  of  air  and  sulfur  dioxide  gases. 

IV.  Action  of  enzymes.     E.g.,  the  action  of  emulsin  upon  amygdaline. 

It  might  appear  that  groups  II  and  III  include  all  the  neces- 
sary reactions,  and  that  the  other  two  groups  are  merely  sub- 
divisions of  these,  although  it  appears  difficult  to  classify  the 
phenomena  of  group  I  as  chemical  reactions. 

The  most  obvious  criticism  of  such  a  system  of  classification 
is  that  for  example,  groups  II  and  III  do  not  appear  to  be 
dependent  upon  reactions  as  catalytic  but  are  applicable  to  all 
chemical  reactions.  No  special  criterion  of  catalysis  is  involved. 

The  question  of  the  part  played  by  a  solvent  in  connection 
with  a  reaction  taking  place  between  dissolved  substances  has 
been  discussed  frequently.  The  velocity  of  a  chemical  change 
may  be  altered  considerably  because  of  the  specific  nature  of 
the  solvent.  The  changes  occurring  in  aqueous  solutions  furnish, 
of  course,  the  most  common  and  best  known  examples.  Other 
solvents  may,  however,  exert  specific  actions  as  well.  Thus,  the 
rate  of  transformation  of  ammonium  cyanate  into  urea  was 
found  to  be  thirty  times  as  great  in  ethyl  alcohol  as  in  water.2 
Ostwald3  clearly  recognized  the  fact  that  solvents  should  be 
regarded  as  catalysts  and  that  there  is  no  difference  in  principle 
between  a  small  amount  of  substance  acting  as  a  catalyst,  or  a 
large  amount  (as  with  a  solvent).  Van't  Hoff,4  however,  includ- 
ing the  criterion  of  unchanged  equilibrium  in  his  description  of  a 
catalyst,  pointed  out  that  the  effect  of  a  medium,  such  as  the 
solvent,  on  a  catalytic  action  may  make  itself  evident  in  one 
of  two  ways.  In  the  first  place,  the  solvent  (or  medium)  may 
affect  the  two  opposing  actions  of  a  reversible  reaction  in  the 

1  W.  Ostwald,  Z.  Elektrochem.  7,  995   (1901)  ;  Nature  65,  522   (1902). 

2  J.  Walker  and  S.  A.  Kay,  J.  Ghent.  Soc.  71,  489    (1897). 

3  W.  Ostwald,  Z.  Elektrochem.  7,  998   (1901). 

4  J.  H.  van't  Hoff,  "Lectures  on  Theoretical  and  Physical  Chemistry,"  trans- 
lated by  R.  A.  Lehfeldt,  1899,  Vol.  1,  p.  221, 


INTRODUCTION:    GENERAL  VIEWS  ON  CATALYSIS  21 

same  way  and  consequently  have  no  influence  on  the  final  state 
of  equilibrium;  and  in  the  second  place,  the  solvent  may  exert 
a  specific  action  depending  upon  the  relation  between  the  solvent 
and  each  of  the  reacting  substances.  Van't  Hoff  showed  how 
the  disturbing  effect  of  the  solvent  on  the  state  of  equilibrium 
might  be  eliminated  theoretically. 

The  possible  action  of  the  solvent  as  a  catalyst  has  been  indi- 
cated because  of  the  number  of  reactions  which  occur  in  solu- 
tions, and  to  bring  out  the  fact  that  whether  or  not  the  solvent 
is  assumed  to  act  as  a  catalyst,  and  the  consequent  theoretical 
deductions,  depends  essentially  upon  the  definition  of  catalyst 
which  is  adopted.  This  fact  will  appear  in  other  connections 
as  well. 

Catalytic  actions  are  extremely  common.  Ostwald  stated 
that  "There  is  probably  no  kind  of  chemical  reaction  which  can- 
not be  influenced  catalytically  and  there  is  no  substance,  element 
or  compound,  which  cannot  act  as  a  catalyzer."  The  wide- 
spread character  of  the  chemical  changes  which  have  been  in- 
cluded under  catalytic  reactions  accounts  for  the  difficulty  of 
developing  a  simple  classification  of  catalytic  actions  which 
would  be  different  from,  or  at  least  not  dependent  upon,  the  clas- 
sification of  chemical  reactions  in  general.  A  number  of  theories 
of  catalytic  actions  have  been  suggested  at  various  times  deal- 
ing with  separate  groups  of  reactions  and  accounting  for  these 
in  a  more  or  less  satisfactory  but  isolated  manner.  No  general 
viewpoint  appears  to  have  been  developed  in  this  way  which 
would  include  catalytic  phenomena  as  a  whole,  separate  from, 
even  if  related  to,  other  chemical  reactions.  It  seems  to  the 
writer  that  a  more  satisfactory  result  might  be  obtained  if  it 
were  possible  to  develop  a  viewpoint  which  would  include  all 
chemical  reactions,  including  those  classed  heretofore  as  cata- 
lytic, without  calling  attention  primarily  to  the  special  attributes 
which  are  taken  to  predominate  in  the  classification  of  catalytic 
reactions.  This  would  mean  that  the  mechanism  of  chemical 
reactions  in  general  is  the  main  factor  involved,  that  certain 
regularities  and  principles  applicable  to  all  reactions  might  be 
developed  from  such  a  study,  and  that  catalytic  reactions  would 
form  a  special  group  in  such  a  system  in  which  certain  conditions 
may  be  fixed,  but  where  the  phenomena  at  the  same  time  obey 


22  CATALYTIC  ACTION 

the  general  laws  observed  with  chemical  reactions  as  a  whole. 
This  view  will  not  be  developed  farther  here  but  will  be  brought 
out  in  greater  detail  in  Chapter  III. 

The  industrial  applications  of  catalytic  actions  can  only  be 
referred  to  briefly.  The  catalytic  reactions  which  have  been 
used  and  are  being  used  in  this  way  are  so  numerous  that  a  men- 
tion of  a  few  of  those  which  are  specially  prominent  at  the  pres- 
ent time  is  all  that  can  be  attempted.  Thus,  the  various  proc- 
esses of  sulfuric  acid  manufacture,  including  the  contact  process, 
the  Haber  and  the  Claude  ammonia  processes,  the  various 
hydrogenation  processes  for  fats  and  oils,  the  oxidation  reactions 
such  as  the  conversion  of  ammonia  into  nitric  acid,  the  Deacon 
process  for  chlorine,  and  the  production  of  phthalic  acid  from 
naphthalene  by  sulfuric  acid  in  the  presence  of  mercury  or  other 
metals,  the  saponification  of  oils  (Twitchell  process),  the  dehy- 
dration of  alcohol  to  form  under  certain  conditions  unsaturated 
hydrocarbon  and  under  different  conditions  ether,  and  the  com- 
bustion reactions  taking  place  in  the  presence  of  mixtures  of 
various  metallic  oxides  such  as  those  used  in  incandescent  gas 
mantles,  are  a  few  in  which  catalytic  reactions  in  one  way  or 
another  play  a  predominating  role.  Certain  enzyme  actions  are 
also  to  be  included.1  Reference  will  be  made  in  the  following 
chapters  to  the  phenoihena  occurring  in  some  of  these  reactions, 
not  from  the  point  of  view  of  their  industrial  significance,  but 
rather  as  bearing  upon  certain  questions  of  general  interest  in 
the  classification  and  explanation  of  the  reactions. 

1  Cf.  K.  G.  Palk,  "The  Chemistry  of  Enzyme  Actions,"  Chapter  IX,  "Uses 
and  Applications  of  Enzymes." 


Chapter  II. 
Reaction  Velocity  and  Catalysis. 

It  was  stated  in  Chapter  I  that  the  outstanding  advance  in 
the  development  of  the  views  on  catalysis,  following  the  pioneer 
work  of  Berzelius  was  the  introduction  of  the  concept  of  reaction 
velocity  by  Ostwald.  Ostwald  defined  catalysis  as  the  accelera- 
tion of  a  chemical  reaction  by  the  presence  of  a  substance  which 
is  itself  unchanged  as  a  result  of  the  reaction.  The  theoretical 
bases  of  reaction  velocity  had  been  given  a  firm  foundation  by 
the  study  of  the  kinetics  of  a  number  of  chemical  reactions  and 
the  application  of  mathematical  equations,  in  the  integrated 
as  well  as  in  the  differential  forms,  to  represent  the  velocities 
under  different  conditions.  The  possibility  of  considering  cata- 
lytic actions  from  a  quantitative  point  of  view  marked  a  dis- 
tinct advance  in  the  treatment  of  these  reactions.  In  this  chap- 
ter, an  attempt  will  be  made  to  present  some  of  the  principles 
upon  which  chemical  kinetics  are  based,  and  certain  limitations 
in  their  application  to  catalysis. 

Before  doing  this  it  may  be  pointed  out  that  the  view  of 
change  in  reaction  velocity  has  been  recognized  by  most  workers 
as  an  essential  part  of  catalysis.  Thus  Bredig x  considered 
catalysis  to  consist  of  the  acceleration  of  a  slow  chemical  reac- 
tion by  means  of  the  presence  of  a  foreign  body.  Stieglitz 2 
went  farther.  He  considered  that  the  only  fundamental  fact 
common  to  all  catalytic  actions,  or  the  characteristic  of  such 
actions,  is  an  acceleration  in  the  rate  of  reaction  and  that  further 
deductions  in  connection  with  catalysis  are  merely  applications 
of  the  fundamental  laws  of  chemistry.  The  views  presented  in 
these  contributions  will  be  taken  up  again  later  in  this  chapter. 

With  regard  to  the  general  kinetic  relationships  which  have 

1  G.  Bredig,  Ergebniase  der  Physiologic,  I,  134    (1902). 

2  J.  Stieglitz,  Proc.  Congr.  Arts  and  Sciences,  St.  Louis,  1904,  Vol.  IV,  pp. 
276-84;  Am.  Chem.  J.  39,  63   (1908). 

23 


24  CATALYTIC  ACTION 

been  developed  and  applied  with  a  number  of  reactions,  this  is 
not  the  place  to  enter  into  the  details.  Such  expositions  may  be 
found  in  a  number  of  publications  devoted  more  specifically  to 
these  problems.  Reference  can  be  made  here  only  to  such  ques- 
tions as  the  numbers  of  molecules  taking  part  in  reactions  or  the 
orders  of  the  reaction  including  methods  of  their  determination, 
the  influence  of  external  factors  such  as  temperature  and  pres- 
sure, etc.  At  the  same  time,  it  must  be  emphasized  that  for  a 
proper  and  satisfactory  understanding  of  the  relations  involved, 
the  mathematical  deductions  and  equations  are  absolutely  essen- 
tial. They  are  not  repeated  here,  except  for  the  purpose  of  bring- 
ing out  definite  points,  because  of  the  necessary  limitations  of 
space,  since  an  incomplete  presentation  is  even  less  satisfactory 
than  none  at  all. 

In  order  to  develop  some  fundamental  points  in  connection 
with  chemical  kinetics,  the  following  brief  outline  will  be  pre- 
sented. In  the  development  of  the  mathematical  equations  rep- 
resenting the  kinetics  of  chemical  reactions,  the  general  expres- 
sion may  be  put  in  the  form 

v  =  k  G!  c2 cn,  (1) 

in  which  v  represents  the  velocity  of  the  reaction,  or  the  change 
in  concentration  of  the  substances,  c17  c2,....  in  unit  time 
at  constant  temperature,  each  c  term  representing  one  of  the  sub- 
stances undergoing  change.  The  symbol  v  would  be  given  in 
more  exact  mathematical  form  as  the  differential  expression 

—  _  which  represents  the  rate  of  change  of  the  concentration  of 
at 

one  or  more  of  the  substances  clf  c2, The  term  k  represents 

the  proportionality  constant,  characteristic  for  the  reaction  in 
question  under  the  given  conditions.  The  integrated  expression 
of  equation  (1)  (as  a  differential  equation)  is  used  in  the  prac- 
tical tests  in  chemical  kinetics. 

Equation  (1)  and  the  expressions  derived  from  it  are  based 
primarily  upon  the  law  of  mass  action.  This  raises  at  once 
several  pertinent  questions.  Some  of  these  were  taken  up  in 
another  connection.1  The  following  quotations  will  serve  to 

*"The  Chemistry  of  Enzyme  Actions,"  K.  G.  Falk,  American  Chemical 
Society;  Monograph  Series.  The  Chemical  Catalog  Co.,  Inc.,  New  York,  1921. 
Chapter  II. 


REACTION  VELOCITY  AND  CATALYSIS          25 

illustrate  the  significance   of  the  theoretical   basis  underlying 
equation  (I)1 

"The  law  of  mass  action  forms  the  basis  of  the  exact  study  of  chemical 
kinetics.  This  law  states  that  the  amount  of  substance  undergoing  change 
in  a  unit  of  time  is  proportional  to  the  active  mass  present  during  that 
time.  This  law  is  of  general  applicability.  In  applying  it  in  chemical 
reactions,  it  is  obviously  necessary  to  use  certain  units  of  mass  or  quantity 
in  order  to  determine  the  active  mass  of  substance  present  at  any  given 
time.  The  simplest  view  to  take  is  that  the  active  mass  of  a  substance  is 
given  by  its  molecular  concentration.  For  practical  purposes,  therefore, 
the  number  of  gram  molecules  or  mols  per  liter  of  volume  will  be  used 
as  the  active  mass. 

"Before  going  further,  however,  it  is  necessary  to  emphasize  the  simpli- 
fication which  has  been  introduced.  The  active  masses  have  been  replaced 
by  molecular  concentrations,  and  therefore,  the  law  of  mass  action  has 
been  changed  to  the  law  of  concentration  action.  If,  now,  deductions  from 
the  law  of  concentrations  are  found  not  to  be  valid,  this  does  not  mean 
that  the  law  of  mass  action  does  not  hold,  but  that  an  incorrect  hypothesis 
may  have  been  introduced  in  the  substitution  of  concentrations  for  active 


The  relation  between  the  chemical  equation  and  the  mathe- 
matical equation  deserves  consideration.  The  chemical  reaction 
may  be  represented  by  an  equation  of  the  form 

A  +  B  +  ....=L  +  M+....  (2) 

This  equation  indicates  the  transformation  of  the  substances 
whose  chemical  compositions  are  given  by  A,  B,  . . . .  into  the 

substances  whose  chemical  compositions  are  given  by  L,  M}. 

and  the  relative  amounts  of  the  substances  changed.  Nothing 
is  shown  by  the  chemical  equation  beyond  this.  The  rate 
of  reaction,  the  energy  change,  and,  in  general,  the  mechan- 
ism of  the  chemical  reaction  is  not  shown  by  the  chemical  equa- 
tion. The  chemical  equation  as  ordinarily  written  only  shows 
the  substances  whose  chemical  compositions  have  been  changed 
and  the  chemical  compositions  of  the  resulting  substances.  Any 
number  of  substances  not  shown  in  the  chemical  equation  may 
have  taken  part  in  the  chemical  reaction,  but  as  long  as  their 
chemical  compositions  are  unchanged  in  the  final  state  of  the 
reaction,  they  are  not  included  in  the  chemical  equation.  Also, 
the  chemical  equation  ordinarily  does  not  show  the  mechanism 
of  the  reaction,  whether  the  reaction  takes  place  in  two  or  more 
stages,  possibly  involving  other  substances  without  changing 
their  compositions. 

1  L.  c.  p.  20-21. 


26  CATALYTIC  ACTION 

These  limitations  of  the  chemical  equations  must  be  borne 
in  mind  in  discussing  the  theoretical,  and  also  the  practical, 
aspects  of  the  mechanisms  and  the  velocities  of  chemical  reac- 
tions. The  chemical  equations  show  only  the  compositions  of 
the  changing  substances  and  the  masses  involved.  In  combining 
the  chemical  equations  with  the  kinetic  equations,  these  limita- 
tions must  be  included  in  the  deductions  and  conclusions. 

The  limitations  inherent  in  the  original  expression  must  neces- 
sarily be  found  in  the  final  integrated  equation.  The  chemical 
limitation  is  that  of  considering  only  the  substances  whose  com- 
positions finally  are  different  from  the  compositions  of  the  sub- 
stances initially.  Other  substances  are  not,  as  a  rule,  repre- 
sented in  the  chemical  equations,  and,  consequently,  are  not 
included  in  the  kinetic  equations.  This  leads  at  once  to  several 
important  reservations  in  the  use  of  the  chemical  equations.  It 
is  readily  conceivable  that  a  substance  takes  part  in  a  reaction 
but  retains  the'  same  composition  at  the  end  of  the  reaction  as  it 
had  at  the  beginning.  Such  reactions  will  be  referred  to  fre- 
quently in  this  book.  For  one  thing,  the  great  group  of  reac- 
tions which  take  place  in  solvents  such  as  water,  may  be  quoted. 
In  the  absence  of  the  water,  the  reactions  apparently  do  not  take 
place;  in  its  presence,  they  take  place  readily.  The  water  must 
play  some  part,  but  it  is  generally  omitted  in  writing  the  chemi- 
cal equation.  Also,  one  of  the  criteria  of  a  catalyst  is  an  un- 
changed composition,  and  such  reactions  would  therefore  be 
included  in  this  group.  This  question  will  be  taken  up  in  more 
detail  in  the  following  chapter. 

Closely  connected  with  these  factors  is  the  possibility  of  the 
reaction  taking  place  in  two  or  more  stages.  Here,  if  one  of 
the  reaction  steps  takes  place  rapidly  and  the  other  slowly,  it 
will  be  the  velocity  of  the  slow  reaction  which  is  being  measured 
experimentally,  although  the  chemical  analyses  of  the  initial 
and  final  products  would  throw  no  light  on  the  relative  velocities 
in  the  two  stages.  The  kinetic  equation  would  then  give  values 
for  the  constant  indicating  that  the  reaction  was  proceeding 
according  to  the  mechanism  assumed  in  the  complete  chemical 
equation  when  actually  only  the  slowest  step  of  a  series  of  suc- 
cessive changes  is  being  measured  by  the  kinetic  equations.  This 
is  unquestionably  true  for  almost  all  reactions  which  take  place 


REACTION  VELOCITY  AND  CATALYSIS          27 

in  aqueous  solution  where  the  water  is  not  assumed  to  take  part 
directly  in  the  actions. 

One  point  must,  however,  be  mentioned  in  this  connection. 
If  the  concentration  of  one  of  the  reacting  constituents  present 
during  a  reaction  is  so  large  that  during  the  period  of  time  that 
the  reaction  is  under  observation  the  change  in  its  concentration 
is  small  compared  with  the  amount  present,  then  the  application 
of  the  kinetic  equations  will  apparently  indicate  that  it  is  not 
taking  active  part  in  the  reaction.  This  means  that  the  experi- 
mental methods  under  such  conditions  will  not  reflect  the  (com- 
paratively) small  change  in  concentration  of  this  constituent. 

To  return  to  equation  (1),  the  constant  k  when  found  by  the 
use  of  the  suitable  integrated  kinetic  expression  from  a  series 
of  experimental  measurements  of  a  chemical  reaction  at  different 
intervals  during  its  course,  is  taken  to  be  characteristic  for  this 
reaction  under  the  definite  conditions  at  the  temperature  in 
question.  Rise  in  temperature  increases  the  value  of  k  greatly; 
two  to  three-fold  for  a  10°  increase.  Any  change  in  the  reaction 
mechanism  or  the  conditions  under  which  the  reaction  is  taking 
place  would  show  itself  in  a  changing  value  of  k.  In  general, 
a  constancy  of  k  as  determined  experimentally  under  definite  con- 
ditions is  taken  to  be  satisfactory  evidence  that  the  reaction  in 
question  follows  the  course  assumed  in  the  chemical  equation. 
If  a  change  in  phase  occurs  in  the  course  of  the  reaction,  such 
as  a  gas  being  evolved  or  a  solid  precipitated,  or  if  a  reaction 
takes  place  at  the  boundary  of  two  phases,  entirely  misleading 
conclusions  may  be  drawn  from  a  consideration  of  the  kinetic 
results.  Under  such  conditions,  the  measurements  often  repre- 
sent only  a  diffusion  rate,  and  the  mathematical  expression  found 
to  hold  would-be  similar  in  form  to  that  representing  a  mono- 
molecular  reaction  rate. 

The  definition  of  Ostwald  of  a  catalytic  action  involves  an 
increase  in  velocity  by  the  presence  of  a  substance  which  is 
unchanged  after  the  reaction.  Referring  to  equation  (1),  this 
means  an  increase  in  v.  This  increase  in  v  necessitates  a  change 
in  k  or  in  the  concentration  terms  or  in  both.  By  hypothesis,  the 
chemical  equation  is  unchanged,  therefore  an  increase  in  the 
value  of  k  by  the  addition  of  a  substance  which  does  not  appear 
in  the  products  would  denote  a  catalytic  action,  the  temperature 


28  CATALYTIC  ACTION 

remaining  constant.  The  added  substance,  unchanged  after  the 
reaction,  is  the  catalyst.  Nothing  is  postulated  with  regard  to 
the  mechanism  of  the  action  of  the  catalyst. 

Before  going  farther,  it  is  necessary  to  consider  the  increase 
in  the  value  of  k  more  carefully.  There  are  two  ways  in  which 
this  increase  may  appear.  These  two  ways  have  not  been  con- 
sidered separately  heretofore  as  far  as  the  writer  is  aware,  but 
have  always  been  used  interchangeably  in  speaking  of  catalysis 
in  general,  although  in  the  consideration  of  individual  reactions 
at  one  time  one  way  is  specified,  at  another  time,  the  other. 

In  the  first  place,  the  presence  of  a  catalyst  may  increase  the 
velocity  of  a  chemical  reaction  by  a  definite  amount.  Thus,  in 
the  absence  of  the  catalyst,  k  would  have  a  certain  constant 
value;  in  its  presence,  k  would  also  be  constant,  but  would  be 
greater  in  magnitude.  The  new  value  of  k  would  be  as  charac- 
teristic for  the  reaction  with  the  given  concentration  of  the 
catalyst,  as  the  smaller  value  was  for  the  reaction  in  the  absence 
of  the  catalyst.  Perhaps  the  best  known  example  of  this  type 
of  catalysis  is  the  hydrolysis  of  sucrose  in  the  presence  of  acids 
of  different  strengths,  the  rates  being  dependent  within  limits 
upon  the  concentrations  of  the  hydrogen  ions  which  are  taken 
to  act  as  catalysts. 

In  the  second  place,  the  presence  of  a  catalyst  in  certain 
reactions  causes  an  acceleration  of  the  reaction;  that  is  to  say, 
with  a  certain  amount  of  catalytic  substance  added  initially, 
the  velocity  of  the  chemical  transformation  increases  continu- 
ously. Numerous  examples  of  this  type  of  catalysis  are  known. 
Thus,  the  formation  of  ethyl  acetacetate  from  ethyl  acetate  may 
be  written  as  follows: 

2CH3CO .  OC2H5  =  CH3CO .  CH2CO .  OC2H5  +  C2H5OH.  (3) 

It  was  found  that  a  very  small  amount  of  ethyl  alcohol  (or 
better  sodium  ethylate)  was  needed  to  have  this  reaction  take 
place  at  an  appreciable  rate  but  that  after  it  had  started,  the 
rate  increased  continually.  The  alcohol,  or  alcoxide,  evidently 
acted  as  catalyst,  and  the  alcohol  formed  as  the  reaction  pro- 
ceeded, itself  then  played  a  part  as  catalyst.  The  reason  for 
the  acceleration  in  this  case  is  therefore  to  be  ascribed  to  the 
continually  increasing  quantity  of  catalyst.  It  is  well  known, 


REACTION  VELOCITY  AND  CATALYSIS  29 

of  course,  that  the  explanation  commonly  accepted  for  the 
mechanism  of  this  reaction  involves  several  successive  steps. 
This  will  be  referred  to  again  later.  Another  set  of  reactions 
belonging  to  this  class  includes  the  hydrolysis  of  organic  esters 
by  acids  not  too  highly  ionized.  Here,  also,  one  of  the  products 
formed  as  the  result  of  the  reaction,  acid  in  this  case,  acts  as 
catalyst  increasing  the  speed  of  the  hydrolysis.  A  third  reaction 
which  may  be  mentioned  is  the  action  of  permanganate  on  oxalic 
acid,  a  method  for  the  quantitative  estimation  of  the  latter.  The 
presence  of  a  manganous  salt  increases  the  velocity  of  the  reac- 
tion, and  since  in  the  reaction  the  manganese  of  the  permanga- 
nate is  transformed  into  such  a  salt,  the  amount  of  the  latter 
continually  increases  and  the  reaction  is  constantly  speeded  up. 

These  two  groups  of  velocity  changes  which  are  included  un- 
der the  Ostwald  definition  of  catalysis  can  be  differentiated 
sharply.  The  second  group,  including  accelerated  reactions  in 
which  the  changes  are  connected  with  the  behaviors  of  one  or 
more  of  the  products  of  the  reactions  exerting  catalytic  actions, 
have  also  been  termed  "auto-catalytic"  reactions. 

While  the  two  types  of  velocity  changes,  finite  increase  and 
accelerated  increase,  can  be  differentiated  sharply  in  any  given 
experimental  reaction,  a  closer  analysis  of  the  phenomena  shows 
that  fundamentally  the  same  actions  as  regards  the  catalytic 
nature  of  the  changes  are  involved.  In  the  first  case,  the  addi- 
tion of  a  definite  quantity  of  the  catalytic  substance  to  a  reac- 
tion results  in  a  certain  increase  in  the  velocity  of  the  chemical 
change.  The  mechanism  by  means  of  which  the  catalyst  causes 
this  increase  is  not  of  special  significance  in  the  present  discus- 
sion except  that  the  catalyst  must  be  involved  in  some  way  in 
the  reaction.  In  the  second  case  of  accelerated  change  in 
velocity,  or  auto-catalysis,  as  the  chemical  reaction  proceeds  the 
substance  which  plays  the  part  of  catalyst  is  formed  continu- 
ously as  one  of  the  products  of  the  chemical  change.  The  con- 
centration of  the  catalyst  therefore  is  continually  increasing. 
The  mechanism  of  the  action  of  a  definite  amount  of  the  catalyst 
in  the  second  case  may  be  considered  to  be  similar  to  that  of 
the  mechanism  of  the  action  of  the  catalyst  in  the  first  case. 
Whatever  explanation  is  adopted  for  one  of  the  actions  can  be 
carried  over  to  the  other.  There  is  no  reason  to  consider  the 


30  CATALYTIC  ACTION 

mechanisms  as  different.  The  actual  amount  of  change  under- 
gone in  any  small  unit  of  time  in  either  case  will  be  dependent 
upon  the  concentration  of  the  catalyst  at  that  time.  In  the  first 
case  the  concentration  of  the  catalyst  does  not  change  through- 
out the  reaction,  in  the  second  it  is  present  in  increasingly 
greater  concentration  as  the  reaction  proceeds.  The  latter  reac- 
tion might  therefore  be  considered  to  be  made  up  of  a  great 
number  of  separate  reactions  in  each  of  which  the  concentration 
of  the  catalyst  is  greater  than  in  the  preceding  reaction,  but 
where  each  reaction  strictly  speaking  may  be  classed  as  a  reac- 
tion of  the  former  type.  The  view  of  the  whole  series  of  reac- 
tions would  give  the  reaction  of  the  second  type,  that  of  accel- 
erated change.  The  two  types  of  reaction  velocity  increases  are 
thus  seen  to  be  based  fundamentally  upon  the  same  relations  or 
changes,  although  experimentally  they  are  readily  grouped 
separately. 

The  definition  of  catalysis  as  involving  an  increase  in  the 
velocity  of  a  chemical  reaction  is  clear  cut,  and  while  perhaps 
limited  in  scope,  gives  a  satisfactory  basis  for  the  classification 
of  such  reactions.  Unfortunately,  the  definition  did  not  remain 
as  simple  as  this  for  any  length  of  time.  Ostwald,  followed  by 
Bredig,  Stieglitz,  and  others,  extended  the  view  so  that  the 
change  in  velocity  might  be  considered  to  be  negative  in  sign 
as  well  as  positive,  or  that  catalysis  includes  retardation  as  well 
as  increase.  This  negative  phenomenon  was  termed  negative 
catalysis.  Innumerable  examples  might  be  given,  and  the  reac- 
tions divided  into  groups  as  with  catalysis;  a  definite  decrease 
in  velocity,  and  an  accelerated  decrease  in  velocity  (negative 
auto-catalysis  or  auto-retardation).  Examples  of  the  former 
include  the  actions  of  certain  organic  substances  (mannite,  ben- 
zaldehyde,  etc.)  on  the  oxidation  of  sodium  sulfite  by  oxygen,1 
the  action  of  various  substances  in  retarding  the  decomposition 
of  hydrogen  peroxide  solutions,  etc.;  while  a  striking  example 
of  the  latter  is  the  action  of  the  hydrobromic  acid  which  is 
formed  in  the  hydrolytic  decomposition  of  bromopropionic  acid 
on  this  decomposition.2 

These  negative   catalysis   phenomena   have   been   explained 

!S.  L.  Bigelow,  Z.  physik.  Chem.  26,  493   (1898). 

2G.  Senter,  J.   Chem.  Soc.  95,  1827    (1909)  ;   G.   Senter  and  A,  W.   Porter 
/.  Chew.  Soc.  99 ,  104£  (191}). 


REACTION  VELOCITY  AND  CATALYSIS  31 

most  simply  by  the  assumption  of  combination  of  the  negative 
catalyst  with  one  or  more  of  the  substances  involved  in  the  reac- 
tion, in  this  way  decreasing  its  active  concentration.  Experi- 
mental evidence  to  support  this  view  is  available  in  a  number 
of  cases.  This  question  will  be  developed  farther  in  a  later 
chapter. 

Referring  again  to  equation  (1),  the  definition  of  catalysis 
would  include  the  phenomena  in  which  the  value  of  k  is  changed 
(increased  or  decreased)  by  the  presence  of  a  substance  which 
is  itself  unchanged  as  a  result  of  the  reaction.  The  change  in  k 
indicates  a  change  in  the  velocity.  By  definition,  this  change 
may  be  negative  as  well  as  positive  and  is  due  to  the  presence 
of  some  substance  which  is  unchanged  as  a  result  of  the  reaction 
(as  shown  in  the  chemical  equation).  This  substance  obviously 
must  produce  some  action  or  be  responsible  for  some  phenomenon 
in  the  reaction,  otherwise  the  velocity  could  not  be  increased 
or  decreased.  Now,  since  the  change  in  velocity  may  be  posi- 
tive or  negative  because  of  some  influence  or  action  of  the  added 
substance,  it  may  be  asked  whether  the  change  in  velocity  might 
not  conceivably  be  zero.  That  is  to  say,  the  effect  of  the 
catalyst  would  show  itself  in  a  velocity  change  ranging  from 
positive,  through  zero,  to  negative.  This  change  in  velocity  is 
a  result  of  the  action  or  influence  of  the  catalyst.  With  zero 
change  in  velocity  for  certain  reactions  it  is  logically  conceivable 
for  a  catalyst  to  be  exerting  an  influence  similar  to  that  exerted 
in  other  reactions  where  the  velocity  is  found  to  change.  In 
other  words,  folldwing  the  description  of  change  in  velocity  as 
characteristic  of  catalysis  one  step  farther,  leads  to  the  conclu- 
sion that  there  must  be  some  underlying  phenomenon  responsible 
for  the  actions  and  that  the  change  in  velocity  is  only  an 
accompanying  phenomenon  and  dependent  upon  the  former.  A 
possible  explanation  of  this  underlying  cause  and  a  grouping 
of  catalytic  actions  as  part  of  a  general  classification  and  inter- 
pretation of  chemical  reactions  will  be  presented  in  the  follow- 
ing chapter. 

It  may  be  stated  that  as  long  as  catalysis  is  assumed  to 
include  only  an  increase  in  reaction  velocity  the  classification 
could  be  developed  in  a  satisfactory  manner,  although  only  a 
narrow  field  would  be  covered,  Extending  the  view  to  include 


32  CATALYTIC  ACTION 

"negative"  catalysis  broadened  the  field,  brought  out  many  more 
analogies,  but  also  showed  that  the  definitions  could  not  be  ad- 
hered to  strictly,  since  a  consistent  following  out  of  the  views  led 
to  conclusions  indicating  the  necessity  for  a  more  fundamental 
definition  of  catalysis. 

It  was  stated  that  the  change  in  velocity  of  a  chemical  reac- 
tion by  the  presence  of  a  substance  which  is  unchanged  at  the 
end  of  the  reaction  would  mean  a  change  in  k  or  in  the  concentra- 
tions of  the  reacting  substances  or  in  both.  The  chemical  equa- 
tion was  assumed  to  be  unchanged  in  the  preceding  discussions 
and  the  change  localized  in  k.  Some  consideration  may  be  de- 
voted to  the  other  possibility,  a  change  in  the  concentrations  of 
the  reacting  substances,  especially  since  the  experimental  and 
theoretical  studies  of  Stieglitz  on  catalysis  have  contributed  much 
valuable  material  to  this  phase  of  the  subject.  His  view,  as 
already  stated,  is  that  "The  one  vital  fact,  then,  of  an  accelera- 
tion due  to  an  increase  in  the  active  mass  or  concentration  of  a 
reacting  component  in  a  catalytic  action  is  the  only  fundamental 
fact  common  to  all  catalytic  actions."  It  is  this  view,  of  increase 
in  the  concentration  of  reacting  component,  which  is  brought  out 
so  strikingly  in  Stieglitz  's  work,  which  it  is  desired  to  emphasize 
here,  as  it  aids  markedly  in  the  development  of  the  chemical 
explanation  of  catalysis,  even  if  his  views  on  acceleration  as  the 
dominating  feature  of  catalysis  are  not  followed. 

Stieglitz  x  studied  the  mechanism  of  a  number  of  hydrolytic 
and  similar  reactions  whose  rates  of  change  were  increased  by 
the  addition  of  acids.  In  place  of  following  the  changes  occur- 
ring in  the  hydrolysis  of  esters,  the  decomposition  of  imido  esters 
was  first  studied  as  furnishing  more  suitable  experimental  ma- 
terial. The  decomposition  of  an  imido  ester,  for  example  methyl 
imido  benzoate,  by  water  may  take  place  as  follows: 

C6H5C  (  :NH)  OCH3  +  H20  =  C6H5C02CH3  +  NH3      (4) 

The  addition  of  an  acid  such  as  hydrochloric  acid  was  found  to 
increase  greatly  the  velocity  of  the  decomposition  of  the  imido 
ester.  In  this  case  the  reaction  was  the  following: 


C6H6C  (  :NH  )  OCH3  +  H20  =  C6H5C02CH3  +  NH       (5) 

1  The  work  was  summarized  in  two  papers  :  J.  Stieglitz,  Jour.  Amer.  Chem. 
800.  32,  221   (1910)  ;  35,  1774   (1913). 


REACTION  VELOCITY  AND  CATALYSIS  33 

The  hydrochlorides,  hydrobromides,  and  nitrates  of  imido  esters 
were  studied.  Taking  into  account  the  degrees  of  ionization  of 
the  imido  ester  salts,  and  allowing  for  the  salt  effect  (increased 
velocity),  the  reaction  velocity  agreed  with  the  view  that  the 
reaction  measured  was  the  decomposition  of  the  positive  ester 
ion.  The  same  rate  of  decomposition  was  found,  irrespective  of 
the  salt  from  which  it  was  derived,  ".  .  .  the  simple  reason  why 
the  addition  of  an  acid  accelerates  this  decomposition  is  that  it 
forms  a  salt  whose  positive  ion  is  the  reacting  component,  and 
that  the  concentration  of  the  ion  is  enormously  increased  when 
the  catalyzing  acid  is  added  to  the  free  ester,  which  is  a  very 
weak  and  therefore  little  ionized  base."  The  reactions  in  all 
probability  take  place  in  stages  as  follows: 

NH*  NHt 

C6H6C  -f-  H  -f-  OH  —  C6H5C — OH     =         (6) 

OCH8  OCH3 

C6H5C02CH3  +  NHt 

in  the  presence  of  acids;  and  with  water  alone  according  to  the 
equations 

NH  NH2 

C6H6G  +  H  +  OH~=  C6H5C— OH     =         (7) 

OCH8  OCH3 

C6H5C02CH3  - 


It  was  pointed  out  that  in  all  this  work,  the  transformation  in 
acid  solution  occurred  in  the  sense  that  the  positive  ion  of  a 
weaker  base  was  transformed  into  the  positive  ion  of  a  stronger 
base.  This  fact  will  be  referred  to  again  in  Chapter  IV.  Imido 
esters  react  with  ammonia  to  form  amidines.  The  rates  of 
these  reactions,  also,  were  increased  by  the  addition  of  acid,  and 
the  main  reacting  components  were  shown  to  be  the  positive  ester 
ions.  This  reaction  may  be  formulated  as  follows': 

C6H5C(:NH2~)OCH3  +  NH8=C6H8C(;NH8~)NH2  +  CH3OH  (8) 


34  CATALYTIC  ACTION 

Similarly,  the  rates  of  the  reactions  between  urea  esters  and 
ammonia  to  form  guanidines  were  increased  by  the  addition  of 
acid,  the  reacting  components  of  the  reactions  which  were  being 
measured  being  the  positive  ester  ions. 

At  the  same  time  that  the  decomposition  of  the  positive  ion 
occurred  in  these  reactions,  water  (or  ammonia)  acted  on  un- 
ionized ester  but  at  a  very  much  slower  rate.  For  example,  for 
the  action  between  ammonia  and  methyl  imido  benzoate,  the 
reaction  involving  the  positive  ester  ion  was  about  50,000  times 
as  rapid  as  that  involving  the  unionized  ester.  At  the  same  time, 
in  observing  the  changes  actually  occurring,  the  unionized  ester 
may  be  present  in  much  greater  concentration  than  the  positive 
ester  ion,  so  that  experimentally  the  amounts  of  the  two  sub- 
stances which  will  have  reacted  (or  the  amounts  of  the  products 
formed  in  the  two  reactions)  will  be  of  the  same  order  of  mag- 
nitude, although  their  rates  differed  widely.  These  relations 
varied  with  the  different  reactions.  With  imido  esters,  the  con- 
centration of  the  positive  ester  ion  in  the  presence  of  acid  was 
comparatively  large.  At  the  other  extreme  is  the  reaction  be- 
tween acid  ester  and  ammonia  to  form  amide,  as  follows: 

CH3C02CH3  +  NH3  =  CH3CONH2  +  CH3OH         (9) 

This  reaction  is  extremely  slow,  even  in  the  presence  of  acids 
or  ammonium  salts,  and  is  essentially  a  function  of  the  ester  and 
the  ammonia.  The  relative  concentrations  of  ester  salt  and 
therefore  positive  ester  ion  and  uncombined  ester  in  any  one  case 
depends  upon  the  relative  affinity  of  the  ester  and  the  other  sub- 
stances present  for  the  acid.  Thus,  in  the  last  case,  the  ester 
forming  an  extremely  weak  oxonium  base  can  take  only  traces 
of  acid  from  the  ammonium  chloride  in  the  presence  of  ammonia. 
The  ester  salt  is  therefore  present  in  very  minute  concentration 
and  the  molecular  transformation,  because  of  the  larger  concen- 
tration of  reacting  components,  comes  to  the  front  experimen- 
tally. In  the  reaction  between  ester  and  water  in  the  presence 
of  acid  to  form  organic  acid  and  alcohol,  the  ester  need  compete 
for  the  acid  only  with  an  oxonium  base  of  the  same  order  of 
strength  or  perhaps  rather  weaker  (oxonium  base  of  water), 
and  therefore  the  ionic  reaction  predominates.  In  any  case  two 
simultaneous  reactions  are  taking  place.  The  actions  of  am- 


REACTION  VELOCITY  AND  CATALYSIS          35 

monia  on  imido  ester  and  on  acid  ester  represent  perhaps  the 
extreme  conditions.  In  the  former,  the  action  involves  essen- 
tially the  positive  ester  ion  and  the  addition  of  catalyst  (acid) 
would  increase  the  velocity  according  to  the  mechanism  out- 
lined ;  in  the  latter,  the  action  involves  essentially  unionized  ester 
and  the  addition  of  acid  to  act  as  catalyst  is  ineffective  for  the 
reasons  given. 

The  hydrolysis  of  esters  and  the  reverse  reactions  of  esterifi- 
cation  in  which  the  velocities  of  the  actions  are  increased  by 
acids  assumes  the  formation  of  complex  oxonium  ion  (ester  or 
acid  plus  hydrogen  ion  of  catalyst)  as  the  chief  reacting  com- 
ponent. The  kinetic  relationships  do  not  permit  of  a  decision 
as  to  whether  the  acid  or  its  derivative  forms  the  oxonium  salt 
and  ion,  or  the  alcohol  or  water.  As  a  result  of  the  imido  ester 
studies,  Stieglitz  concluded  that  the  first  view  was  correct;  that 
the  complex  ion  involved  the  acid  or  its  derivative.  It  may  be 
noted  that  as  a  result  of  a  series  of  extended  studies  including 
the  retarding  effects  of  small  amounts  of  water  either  produced 
or  added  to  the  medium  during  the  progress  of  esterification, 
Goldschmidt,1  while  coinciding  in  the  view  that  complex  salt 
formation  occurs  as  an  intermediate  step  in  esterification  and 
ester  hydrolysis  reactions,  concluded  that  the  alcohol  or  its 
derivatives  formed  the  complex  oxonium  salt  and  ion. 

The  work  of  Stieglitz  on  imido  esters  and  related  compounds 
showed  that  the  increase  in  concentration  of  positive  ion  because 
of  the  addition  of  highly  ionized  acid  increased  the  rate  of  reac- 
tion. The  highly  ionized  acid  was  therefore  called  the  catalyst, 
but  it  was  clearly  shown  that  the  increase  in  concentration  of 
the  positive  ion  accounted  for  the  increased  velocity.  The  changes 
in  velocity  in  these  cases  may  be  referred  to  the  changes  in  the 
concentrations  of  the  reacting  molecular  species.  It  is  evident 
that  if  the  concentration  terms  refer  to  the  total  concentration 
of  the  imido  ester  (or  similar)  molecule,  that  the  change  in 
velocity  would  then  vary  with  the  different  acids  added.  The 
acids  would  then  be  considered  to  be  acting  as  catalysts,  the  con- 
centrations would  be  unchanged,  and  the  change  in  the  velocity 
be  reflected  in  the  change  of  the  value  of  k.  The  careful  experi- 

1H.  Goldschmidt,  Z.  ElektrocJiem.  12,  432  (1906)  :  15,  4  (1909)  ;  Z.  phusik 
fffcfm.  70,  627  (1910)  ;  H.  Goldschmidt  and  O.  Udby,  Z.  physik.  Chem.  60,  728 
(1907). 


36  CATALYTIC  ACTION 

mental  study  of  the  reaction  has  however  shown  that  the  sub- 
stance being  transformed  in  the  reaction  was  the  same  in  every 
case  (namely,  the  complex  positive  ion),  that  the  acid  added 
only  changed  the  proportion  of  this  positive  ion,  and  that  the 
actual  chemical  transformation  of  the  imido  ester  ion  was  identi- 
cal in  every  case.  Inserting  the  proper  concentration  of  positive 
ion  into  the  kinetic  equation  would  result  in  the  corresponding 
value  of  k  being  unchanged.  Until  these  relations  had  been 
worked  out,  the  change  in  velocity  would  have  been  taken  to  be 
accompanied  by  a  change  in  the  constant  k  and  the  reaction 
termed  catalytic  in  the  sense  that  the  action  of  the  added  acid 
was  unknown  except  in  so  far  as  it  increased  the  rate  of  reaction. 

The  problem  appears  to  resolve  itself  into  a  question  of  defi- 
nition. Under  what  conditions  is  a  change  in  velocity  to  be 
referred  to  a  difference  in  the  nature  of  the  chemical  reaction? 
With  regard  to  the  imido  ester  reaction,  the  addition  of  different 
acids  resulted  in  a  different  imido  ester  salt  and  different  degree 
of  ionization.  The  velocity  being  dependent  upon  the  concen- 
tration of  the  ion  might  be  different  with  different  acids  which 
had  been  added.  The  actual  chemical  change  measured,  that 
represented  by  the  chemical  equation  as  the  change  in  imido  ester, 
was  the  same  in  every  case.  At  the  same  time,  it  cannot  be 
denied  that  the  reaction  took  place  in  steps;  formation  of  imido 
ester  salt,  its  ionization  and  the  subsequent  decomposition  of  the 
ester  ion.  The  reaction  measured  kinetically  was  the  last,  the 
slowest  of  the  series  indicated.  These  steps  were  not  indicated  in 
the  chemical  equation,  as  apparently  the  added  acid  was  un- 
changed as  a  result  of  the  reaction.  Strictly  speaking,  the  com- 
plete chemical  reaction  was  different  in  the  different  cases,  and 
the  velocity  might  therefore  be  expected  to  be  different.  At  the 
same  time,  by  choosing  one  of  the  stages  of  the  reaction,  it  was 
possible  to  account  for  the  increase  in  the  velocity  of  the  reac- 
tion by  showing  that  the  concentration  of  the  reacting  constituent 
(as  against  the  total  concentration  of  the  indicated  molecular 
compound)  was  changed  under  different  conditions. 

The  example  of  the  imido  ester  catalysis  has  been  chosen  as 
it  is  a  particularly  instructive  one.  As  Stieglitz  pointed  out,  it 
is  representative  of  a  number  of  reactions.  It  may  be  asked 
whether  in  other  cases,  where,  in  the  kinetic  considerations 
changes  in  velocity  are  observed,  similar  deductions  would  not  be 


REACTION  VELOCITY  AND  CATALYSIS  37 

permissible.  The  actions  of  catalysts,  from  the  minute  quantities 
involved  in  ester  saponifications  and  similar  reactions  to  the 
changes  in  solvents  may  show  themselves  in  changing  the  velocity 
constants  of  the  reactions.  But  the  chemical  equation  which  may 
be  used  to  represent  the  change  may  represent  only  one  of  the 
stages  of  the  reaction,  and  eliminating  even  the  possibility  of 
simultaneous  or  consecutive  reactions  being  measured,  a  suitable 
change  in  the  concentration  of  the  reacting  molecule  or  constit- 
uent frequently  might  account  for  the  action  of  the  so-called 
catalyst. 


Chapter  III. 
Theory  of  Catalytic  Actions. 

The  theory  of  reactions  which  is  based  upon  the  formation  of 
addition  compounds  appears  to  offer  the  most  satisfactory  gen- 
eral viewpoint  from  which  to  consider  the  mechanism  of  chemi- 
cal reactions.  In  the  development  of  this  theory  it  was  found 
that  by  adding  one  simple  concept  and  then  basing  the  deduc- 
tions upon  well  known  laws  and  relations,  a  classification  of 
catalytic  actions  was  obtained  which  included  and  accounted  for 
the  views  heretofore  used  in  that  field  at  the  same  time  that  the 
multiplicity  of  criteria  suggested  at  various  times  was  avoided. 

The  views  on  the  theory  of  the  mechanism  of  chemical  reac- 
tions were  presented  in  somewhat  different  connections  in  two 
recent  publications  by  the  writer  x  and  the  relations  to  a  number 
of  chemical  phenomena  developed.  In  this  chapter  an  outline  of 
the  general  theory  of  chemical  reactions  will  be  given  briefly,  to 
be  followed  by  the  consideration  of  catalytic  reactions  as  a  spe- 
cial group  of  chemical  reactions,  and  finally  some  of  the  points 
of  more  direct  interest  in  catalysis  will  be  developed  at  greater 
length. 

The  addition  theory  of  chemical  reactions  assumes  that  when 
two  or  more  molecules  react,  they  first  combine  to  form  an  addi- 
tion compound  which  then  may  or  may  not  react  farther  to 
break  down  to  form  other  products.  Thus,  for  example,  the 
reaction  between  an  alcohol  and  an  organic  acid  may  be  repre- 
sented as  follows: 

rCH3C02H-]   =  C2H5OH  +  CH3C02H  (a) 

L   C2H5OHj  =  CH3C02C2H6  +  H20  (b) 

The  method  of  writing  the  equations  indicates  the  fact  that  the 
configuration  of  the  intermediate  compound,  or  the  linkings  of 

1  "Chemical  Reactions  ;  Their  Theory  and  Mechanism." 
"The  Chemistry  of  Enzyme  Actions,"  Chapter  III. 

38 


THEORY  OF  CATALYTIC  ACTIONS  39 

the  atoms  in  it,  is  not  definitely  known.  Also,  it  is  not  shown 
whether  the  reaction  started  with  one  set  of  products  or  the 
other;  in  other  words,  the  factor  of  reversibility  is  brought  out. 

The  obvious  question  which  arises  in  connection  with  this 
reaction,  and.  in  fact,  with  all  reactions,  is  whether  the  ex- 
perimental evidence  bears  out  this  view  of  the  mechanism  of  the 
reactions.  The  first  point  which  must  be  made  is  that  the  addi- 
tion (or  intermediate)  compound  which  is  assumed  to  be  formed 
appears  in  most  cases  not  to  be  stable  enough  to  be  isolated  and 
identified.  The  exact  proportions  of  the  different  constituents 
present  in  it  frequently  cannot  be  given.  At  the  same  time  definite 
evidence  exists  that  such  compounds  are  formed.  If  such  addi- 
tion compounds  possess  marked  stability,  the  reaction  is  generally 
written  differently  and  the  chemical  equation  divided  into  several 
stages.  This  is  true,  for  instance,  for  the  Grignard  reaction, 
where  certain  of  the  addition  compounds  have  been  isolated. 
Equation  (1),  for  example,  may  be  used  in  a  more  or  less  sym- 
bolical manner.  There  is  considerable  evidence  at  hand  that  in 
esterification  as  well  as  in  the  hydrolysis  of  esters,  addition  com- 
pounds are  formed,  but  the  composition  of  such  compounds, 
whether  containing  two  molecules  of  acid  to  one  of  alcohol,  two 
of  alcohol  to  one  of  acid,  or  whether  or  not  containing  water,  is 
uncertain,  and  in  fact  may  vary  with  the  different  alcohols  and 
acids.  ,  ,;  [juJJi- 

Equation  (1)  brings  out  certain  additional  relations.  If  none 
of  the  products  or  substances  is  removed  from  the  reaction  mix- 
ture, the  concentrations  of  ihe  various  constituents  present  at 
equilibrium  can  be  obtained  if  the  equilibrium  constants  are 
known.  Since  the  intermediate  compound  would  appear  in  the 
mass  action  expressions  of  both  reactions,  one  constant  would  be 
sufficient  to  determine  the  relative  equilibrium  concentrations. 
If  equilibrium  is  not  attained,  or  if  one  or  more  of  the  substances 
is  removed  from  the  sphere  of  action,  then  the  actual  composi- 
tion of  the  mixture  at  any  instant  would  depend  upon  the  con- 
centrations of  the  various  substances  and  the  velocity  constants 
of  the  reactions.  If  the  separate  substances  shown  either  in  equa- 
tion (a)  or  (b)  were  present  initially,  then  the  observed  velocity 
would  be  made  up  of  the  velocities  of  the  two  separate  reactions. 
If  one  of  these  takes  place  much  more  slowly  than  the  other,  it 


40 


CATALYTIC  ACTION 


would  be  the  velocity  of  the  slow  reaction  which  would  be  ob- 
served experimentally. 

Equation  (1)  represents  a  simple  example  of  the  explana- 
tion (partial,  at  any  rate)  of  the  mechanism  of  a  chemical  reac- 
tion. A  number  of  such  were  given  in  another  place.1  It  is  evi- 
dent that  with  more  complex  substances,  or  where  three  or  more 
substances  are  taking  part,  more  possibilities  for  the  formation 
of  different  sets  of  products  exist.  Thus,  several  examples,  which 
will  be  taken  up  again  in  connection  with  catalytic  actions  later 
in  this  chapter,  may  be  quoted.  The  first  of  these  is  the  reaction 
between  ammonia,  hydrogen  chloride,  and  water,  which  may  be 
written  as  follows: 


NH3 
HC1 
H20 


NH3  +  HC1  +  H20  (a) 

NH4C1  +  H20  (b) 

NH4OH  +  HC1  (c) 

NH3  +  HC1.H20  (d) 


(2) 


An  analogous  reaction  is  that  between  ammonia,  hydrogen  chlo- 
ride, and  platinic  chloride,  which,  omitting  the  possible  partici- 
pation of  water,  may  be  written  as  follows: 


2NH3 
2HC1 

PtCl4 


=  2NH3  +  2HC1  +  PtCl4  (a) 

=  2NH4C1  +  PtCl  4  (b) 

=  (NH3)2PtCl4  +  2HCl  (c) 

=  H2PtCl6  +  2NH3  (d) 


(3) 


As  a  development  of  reactions  (1),  esterification  (or  the  reaction 
in  which  an  alcohol  and  an  organic  acid  take  part)  in  the  pres- 
ence of  an  inorganic  acid  may  be  formulated  in  the  following 
manner : 


C2H5OH  " 
CH3C02H 
HC1 


C2H5OH  +  CH3C02H  +  HC1        (a) 
CH3CO2C2H5  +H20  +  HC1  (b) 

CH3C02H  +  H20          (c) 


(4) 


C2H5C1 


It  is  obvious  that  in  these  formulations,  all  of  the  possible 
reactions  have  not  been  given.  It  would  serve  no  useful  purpose 
to  elaborate  the  theory  in  this  way  in  the  present  instance  since 

1  "Chemical  Reactions  ;  Their  Theory  and  Mechanism."  Cf.  also  K.  George 
Falk  and  J.  M.  Nelson,  Jour.  Amer.  CTiem.  Soc.  37,  1732  (1915). 


THEORY  OF  CATALYTIC  ACTIONS  41 

it  is  only  desired  to  bring  out  the  general  principles  in  this  con- 
nection. With  more  complex  substances  the  possibilities  would 
be  greatly  increased  as  far  as  the  number  of  possible  sets  of  prod- 
ucts are  concerned.  The  actual  products  observed  in  any  case 
would  depend  on  the  relative  velocities  of  the  different  reactions, 
the  concentrations  of  the  substances  and  their  possible  removal 
from  the  sphere  of  action  if  the  mixture  is  not  at  equilibrium,  and 
the  relative  chemical  affinities  as  dependent  on  the  free  energy 
changes  involved  in  the  formation  of  the  different  sets  of  products, 
if  the  reaction  mixture  has  attained  a  state  of  equilibrium. 

A  study  of  the  chemical  literature  of  recent  years  reveals  the 
fact  that  as  the  study  of  the  mechanism  of  chemical  reactions  is 
pursued  more  generally,  an  increasingly  greater  number  of  reac- 
tions is  found  to  conform  to  the  scheme  outlined.  It  would  lead 
too  far  here  to  quote  different  workers  in  this  field,  but  the  gen- 
eral trend  of  the  views  is  apparent. 

In  place  of  starting  with  certain  products  which  go  to  form 
an  addition  compound  which  may  then  decompose  again  into  dif- 
ferent products,  it  is  possible  to  start  with  a  more  or  less  complex 
substance,  or  even  in  some  cases  a  comparatively  simple  sub- 
stance, which  may  decompose  to  form  different  sets  of  products, 
possibly  depending  upon  the  conditions  used  and  the  principle 
of  mass  action.  A  few  examples  of  such  reactions  may  be  quoted, 
and,  in  order  to  include  a  somewhat  different  type,  the  reactions 
chosen  will  include  oxidation-reduction  changes.1  A  simple  re- 
action of  this  type  is  given  by  the  decomposition  of  formic  acid. 

rHroHl=H20  +  CO  (a)  ,., 

|_H<  WjssH.-f  COi  (b) 

Water  and  carbon  monoxide,  reaction  (a),  are  formed  mainly 
when  formic  acid  is  heated  with  sulfuric  or  other  mineral  acids,2 
hydrogen  and  carbon  dioxide  mainly,  reaction  (b),  by  heating 
with  platinum  or  with  finely  divided  rhodium,  ruthenium,  or 
iridium. 

The  reverse  of  reactions  (5)  can  also  be  carried  out;  that  is, 
starting  with  the  products  on  the  right  hand  side  of  the  equation, 
under  suitable  conditions  formic  acid  will  be  formed. 

With  more  complex  bodies,  the  possibilities  in  the  way  of  in- 

1  "Chemical  Reactions ;  Their  Theory  and  Mechanism,"  Chapter  X. 
aCf.  G.  E.  K.  Branch,  Jour.  Amer.  Chem.  Soc.  37,  2316  (1915). 


42  CATALYTIC  ACTION 

creasing  the  numbers  of  sets  of  products  will  obviously  be  in- 
creased, but  the  general  principles  involved  in  determining  the 
course  of  the  reaction  will  remain  the  same. 

Although  the  application  of  the  addition  theory  to  reactions 
of  organic  chemistry  seems  fairly  simple  in  principle,  it  may 
appear  as  if  more  difficulty  would  be  encountered  in  applying  the 
same  general  views  to  reactions  of  inorganic  chemistry,  espe- 
cially reactions  taking  place  in  aqueous  and  similar  solutions, 
in  which  ions  are  involved.  Some  space  may  therefore  be  devoted 
to  this  question.  According  to  the  theory  of  electrolytic  disso- 
ciation, reaction  in  solution  takes  place  as  a  rule  between  ions 
and  because  of  their  presence.  With  the  addition  compound 
theory,  reactions  in  solution  are  not  assumed  to  take  place  be- 
cause of  the  presence  of  ions.  All  combinations  between  atoms  in 
molecules  are  assumed  at  the  present  time  to  be  connected  in 
one  way  or  another  with  the  transfer  or  sharing  of  electrons  be- 
tween or  by  the  atoms.  Combination  is  considered  to  be  electri- 
cal in  character.  As  pointed  out  by  J.  M.  Nelson  and  the  writer 
in  various  publications  on  the  electron  conception  of  valence  and 
related  subjects,  a  simpler  and  in  many  ways  quite  satisfactory 
theory  of  chemical  combination  can  be  developed  by  assuming  a 
transfer  of  an  electron  in  the  production  of  every  chemical  link- 
ing. With  regard  to  the  relation  between  the  extent  of  electro- 
lytic dissociation  and  the  occurrence  of  a  chemical  reaction 
".  .  .  the  readiness  or  speed  with  which  reactions  occurred  was 
a  phenomenon  not  dependent  upon  the  existence  of  ions.  The 
occurrence,  existence,  and  stability  of  ions  in  the  same  way  had 
nothing  to  do  directly  with  the  occurrence  of  chemical  reactions. 
There  is,  however,  an  indirect  connection.  The  physical  property 
shown  by  the  ability  to  conduct  the  electric  current  in  solution 
and  the  chemical  property  shown  by  the  ability  and  readiness  to 
undergo  change  in  composition  alone  or  in  conjunction  with  other 
substances,  are  both  assumed  to  be  due  to  the  same  underlying 
cause.  This  cause,  while  producing  both  effects,  need  not  produce 
both  quantitatively  at  the  same  rate.  That  is  to  say,  under  cer- 
tain conditions,  the  physical  property  would  be  much  the  more 
marked  and  amenable  to  experiment;  under  other  conditions, 
the  chemical  ...  it  is  probable  that  in  solution,  the  property 
of  the  solvent  of  forming  addition  compounds  with  the  dissolved 


TtiBORY  OF  CATALYTIC  ACTIONS  42 

substances  is  the  common  cause  of  the  two  sets  of  phenomena, 
physical  and  chemical.  In  aqueous  solutions,  compounds  of  the 
nature  of  hydrates,  which  have  been  shown  to  exist  in  a  number 
of  cases,  may  well  be  the  cause.  In  some  cases,  such  as  with  uni- 
univalent  salts  in  aqueous  solutions,  very  close  parallelism  exists 
between  the  physical  and  chemical  properties,  with  uni-divalent 
and  more  complex  salts,  the  parallelism  is  not  obvious  or  does  not 
exist  at  all.  Quantitative  proof  of  this  theory  is  not  at  hand, 
but  it  has  been  found  useful  in  the  consideration  of  reactions 
and  will  be  used  here.  To  sum  up  these  relations:  The  changes 
occurring  in  chemical  reactions  do  not  depend  upon  the  electro- 
lytic dissociation  of  the  reacting  substances.  The  chemical 
changes  are  accompanied  very  often  by  electrolytic  dissociation 
phenomena,  but  the  latter  parallel  the  former  (or  vice  versa) 
and  do  not  necessarily  precede  or  cause  them.  The  electron  con- 
ception of  valence  assumes  the  presence  of  excess  electric  charges 
on  all  atoms  existing  in  states  of  combination,  or  the  transfer  of 
valence  electrons  when  atoms  combine.  The  experimental  facts 
of  electrolytic  dissociation  offer  a  method  for  making  some  of 
these  electric  charges  susceptible  to  measurement,  but  electro- 
lytic dissociation  does  not  produce  these  charges  on  the  ions  due 
to  valence  combinations."  x 

The  importance  of  the  solvent  in  reactions  taking  place  in 
solutions  which  show  electrolytic  dissociation  is  the  feature  which 
must  be  emphasized  in  considering  such  reactions  from  the  point 
of  view  of  addition  compound  formation.  J.  M.  Nelson  and  the 
writer  developed  these  views  in  various  directions  some  years 
ago  in  a  paper  entitled  "Electron  Conception  of  Valence.  VII. 
Theory  of  Electrolytic  Dissociation  and  Chemical  Action."2 
The  most  recent  publication  along  certain  of  these  lines  is  a  paper 
by  J.  Kendall  on  "The  Correlation  of  Compound  Formation, 
lonization  and  Solubility  in  Solutions.  Outline  of  a  Modified 
lonization  Theory."  3  Those  interested  in  the  more  detailed  ex- 
position of  these  questions  must  be  referred  to  the  indicated  pub- 
lications, as  it  would  lead  too  far  to  enter  into  greater  detail  in 
the  present  connection. 

1  "The  Chemistry  of  Enzyme  Actions,"  pp.  16-17. 

2  Jour.  Amer.  Chem.  Soc.  37,  1732  (1915). 

1  Proc.  Nat.  Acad.  Sci.  7,  56  (1921)  ;  cf.  also  J.  Kendall  and  P.  M.  Gross, 
Jour.  Amer.  Chem.  Soc.  J^3t  1416  (1921). 


44  CATALYTIC  ACTION 

A  definite  viewpoint  from  which  to  consider  the  mechanism 
of  chemical  reactions  has  now  been  established.  There  is  no  in- 
tention of  holding  that  it  is  the  best  which  can  be  developed  or 
that  it  is  not  subject  to  modification  even  in  its  present  form. 
The  fact  which  it  is  desired  to  emphasize  is  that  the  classification 
is  a  useful  one  in  that  it  includes  all  chemical  reactions  without 
necessitating  more  or  less  arbitrary  distinctions  in  the  funda- 
mental concepts.  It  is  unfortunate  that  the  quantitative  evidence 
available  at  present  to  test  the  views  advanced  is  not  very  exten- 
sive. The  qualitative  evidence  appears  to  support  them  and  it  is 
to  be  hoped  that  the  further  quantitative  work  which  is  being 
carried  on  in  various  directions  in  connection  with  different  phases 
of  the  general  problem  of  chemical  reactions  will  result  in  the 
acquisition  of  sufficient  experimental  data  either  to  bear  out 
the  addition  compound  theory  and  to  modify  it  if  necessary,  or 
to  lay  the  foundations  for  a  more  satisfactory  theory.  For  the 
present,  it  will  be  used  as  outlined  in  the  preceding  pages. 

Catalytic  actions  form  a  group  of  chemical  reactions  and  have 
been  separated  from  other  reactions  in  the  past  because  of  pos- 
sessing certain  characteristics.  It  was  shown  in  the  earlier  chap- 
ters that  the  criteria  which  have  been  used  for  the  purpose  are 
not  altogether  satisfactory.  In  the  publications  by  the  present 
writer  to  which  reference  has  already  been  made,  a  more  definite 
classification  of  catalytic  actions  was  attempted.  This  classi- 
fication will  be  presented  here,  and  also  additional  relations  will 
be  developed  and  an  attempt  made  to  indicate  the  significance 
of  these  relations  in  connection  with  some  of  the  phenomena 
which  have  been  observed  with  chemical  reactions  and  their 
mechanism  in  general. 

In  considering  catalytic  reactions,  the  only  limitation  or  defi- 
nition which  will  be  used  is  that  in  a  chemical  reaction,  the  chem- 
ical composition  of  one  of  the  initial  substances  is  the  same  as 
that  of  one  of  the  products  of  the  reaction.  This  criterion  has 
been  used  in  the  past  as  one  of  the  conditions  which  a  catalytic 
reaction  must  follow,  but  there  have  always  been  other  condi- 
tions which  had  to  be  obeyed.  The  difference  in  the  views  lies 
in  the  fact  that  unchanged  composition  of  one  constituent  is  the 
only  factor  which  is  defined  in  the  present  instance.  All  further 


THEORY  OF  CATALYTIC  ACTIONS  45 

developments  will  follow  from  the  general  principles  of  chem- 
istry and,  especially  in  the  consideration  of  the  mechanisms  of 
the  reactions,  the  principles  already  given.  It  will  be  noted  that 
changes  in  the  velocities  of  reaction  have  not  been  spoken  of  in 
defining  catalytic  reactions.  This  omission  differentiates  the 
views  developed  here  from  those  of  Ostwald,  Bredig,  Stieglitz, 
and  others.  To  the  writer  it  appears  as  if  the  use  of  change  in 
reaction  velocity  as  the  primary  condition  may  lead  to  confu- 
sion, as  already  indicated.  At  the  same  time,  change  in  reaction 
velocity  will  be  considered  in  the  proper  place  in  connection 
with  certain  phenomena  involved  in  catalysis. 

The  substance  whose  composition  is  the  same  before  and 
after  a  chemical  reaction  in  which  it  participates  is  called  the 
catalyst  according  to  the  indicated  view,  and  the  presence  of 
such  a  substance  is  obviously  necessary  for  the  reaction  to  be 
considered  catalytic.  The  participation  of  a  substance  in  a 
chemical  reaction  and  its  appearance  unchanged  in  composition 
in  the  final  products  cannot  readily  be  visualized  unless  the  addi- 
tion theory  of  chemical  reactions,  or  some  similar  theory  is  used. 
To  determine  whether  such  a  substance  has  taken  part  in  the 
reaction,  or  in  other  words  acted  as  a  catalyst,  evidently  requires 
some  change  susceptible  to  experimental  measurement.  The 
most  apparent  changes  which  may  be  observed  are  in  the  first 
place,  a  change  in  the  velocity  of  the  observed  reaction,  and  in 
the  second  place,  the  formation  of  different  products  in  whole 
or  in  part.  The  isolation  of  addition  compounds  of  which  the 
assumed  catalyst  forms  a  part  is  valuable  confirmatory  evi- 
dence, especially  if  the  addition  compound  itself  can  be  shown 
to  react  further  to  produce  the  required  products.  These  changes 
are  secondary  however  to  the  original  condition. 

In  order  to  illustrate  and  develop  the  relation  in  a  more 
definite  way,  certain  examples  may  be  given. 

Equations  (2)  already  quoted  earlier  in  this  chapter  form 
an  instructive  example  and  may  be  repeated  here. 


NH3 
HC1 
H90 


=  NH3  +  HC1  +  H20  (a) 

=  NH4C1  +  H20  (b) 

=  NH4OH  +  HC1  (c) 

HC1.H20  (d) 


(2) 


46  CATALYTIC  ACTION 

Ammonia  and  hydrogen  chloride  do  not  appear  to  react  with 
appreciable  velocity  in  the  dry  state.  Addition  of  small  amounts 
of  moisture  causes  reaction  to  occur.  With  equations  (a)  and 
(b)  (the  addition  compound  is  involved  in  both),  water  would 
be  considered  the  catalyst.  With  reactions  (a)  and  (c),  hydro- 
gen chloride  might  be  considered  the  catalyst  if  this  reaction 
occurred  to  an  appreciable  extent;  with  (a)  and  (d),  ammonia 
might  be  considered  the  catalyst.  The  reaction  observed  in  any 
given  case  depends  upon  the  concentrations  of  the  constituents, 
the  relative  velocities  of  the  different  reactions,  the  removal  of 
one  or  more  of  the  substances  from  the  sphere  of  action,  and  the 
relative  free  energy  changes  of  the  different  reactions  as  already 
outlined,  depending  upon  whether  or  not  the  reactions  have  been 
allowed  to  come  to  equilibrium. 

Equations  (3)  and  (4)  may  be  considered  similarly  and  need 
not  be  repeated. 

The  question  of  change  in  reaction  velocity  on  the  basis  of 
the  suggested  view  of  catalysis  may  be  considered  next.  If  the 
reaction  between  two  substances  to  form  two  different  substances 
takes  place  at  a  definite  rate,  then,  if  a  catalyst  is  added,  the 
course  of  the  reactions  may  be  symbolized  in  the  following 
manner: — 

=  A  +  B  (a) 

=  E  +  F  (b) 

=  A  +  B  +  C  (a) 

—  E  +  F  +  C  (b)  (7) 

=  M+  N  +  C  (c) 


H 


Equations  (6)  indicate  the  changes  in  the  absence  of  catalyst; 
equations  (7)  the  changes  in  the  presence  of  the  catalyst  C. 
The  products  indicated,  aside  from  the  catalyst,  by  reactions 
(a)  and  (b)  in  both  cases  are  the  same;  those  in  equation  (c) 
are  different. 

Limiting  the  discussion  for  the  present  to  the  reactions  (a) 
and  (b),  it  will  be  seen  that  in  comparing  the  rates  of  trans- 
formation of  products  (a)  into  products  (b)  in  the  two  cases, 
three  possibilities  exist.  Addition  compounds,  indicated  by  [D] 
and  by  [H]  are  formed  in  the  two  sets  of  reactions.  In  the  first 


THEORY  OF  CATALYTIC  ACTIONS  47 

set,  equations   (6),  the  addition  compound  is  made  up  of  the 
two  reacting  substances;  in  the  second,  equations  (7),  of  the  two 
reacting  substances  and  catalyst  substance.    The  combination  of 
catalyst  and  one  of  the  reacting  substances  obviously  is  also  pos- 
sible but  will  not  be  considered  at  this  point  since  it  only  com- 
plicates the  problem  needlessly.     Without  the  catalyst   (equa- 
tions (6) )  the  reaction  proceeds  at  a  definite  rate.  With  the  cata- 
lyst present  the  simultaneous  formation  of  the  two  addition  com- 
pounds shown  in  equations  (6)   and   (7)   is  possible,  and  their 
decompositions  evidently  represent  simultaneous  reactions.     If 
the  velocity  of  the  reaction  involving  the  catalyst   (equations 
(7a)-(7b))  is  less  than  that  of  the  reaction  without  the  catalyst 
(equations  (6) )  (actually  tbe  sums  of  the  velocities  of  the  forma- 
tion and  decomposition  of  the  additive  compounds  are  meant), 
then,  if  there  is  a  large  difference,  probably  only  a  small  part,  if 
any,  of  the  reaction  will  follow  the  former  course,  and  the  veloc- 
ity will  be  practically  the  same  in  the  two  cases.  As  the  difference 
between  the  rates  becomes  less,  more  of  the  reaction  will  involve 
the  catalyst  substance   (equations   (7)),  and  the  total  velocity 
observed  may  be  smaller  than  in  its  absence,  especially  if  the 
catalyst  substance  is  present  in  considerable  amount.     If  the 
velocities  of  the  simultaneous  reactions  are  the  same,  no  change 
due  to  the  catalyst  would  be  observable.    This  represents  the 
second  possibility.    For  the  third  possibility,  the  reaction  with 
the  catalyst  is  the  more  rapid  and  will  therefore  be  the  reaction 
whose  rate  is  being  observed  and  measured.    In  considering  these 
three  possibilities,  it  is  evident  that  the  velocity  of  a  chemical 
reaction  will  be  observed  to  be  diminished  under  special  condi- 
tions because  of  the  direct  participation  of  a  catalyst  substance 
in  the  reaction.     In  most  cases  the  velocity  may  be  said  either 
to  be  unchanged  or  increased.     Only  if  there  is  a  change  would 
the  velocity  measurements  give  evidence  for  the  presence  of  a 
catalyst.     Since  a  decrease  is  comparatively  infrequent,  the  rea- 
son is  evident  for  the  belief  that  velocity  increase  is  the  pre- 
dominating factor  in  catalysis.    The  present  point  of  view  shows, 
however,  that  this  increase  is  a  consequence  of  catalytic  action 
and  is  not  the  controlling  feature.    It  is  the  phenomenon  perhaps 
most  readily  observed  as  a  result  of  catalytic  action,  but  it  rep- 
resents only  a  single  case  in  the  way  of  possible  changes. 


48  CATALYTIC  ACTION 

The  application  of  these  views  to  the  decomposition  of  imido 
esters  and  related  compounds  in  the  presence  of  acid  in  aqueous 
solution  as  studied  by  Stieglitz,  which  was  described  in  some  de- 
tail in  Chapter  II,  is  obvious.  The  simultaneous  reactions  there 
involve  the  decompositions  of  the  unionized  molecules  and  of 
the  positive  ester  ions.  The  effects  of  the  relative  rates  of  the 
different  reactions  and  of  the  relative  concentrations  of  the  re- 
acting components  were  shown  to  be  reflected  in  the  relative 
amounts  of  the  products  formed  in  these  simultaneous  reactions. 

In  the  consideration  of  the  mechanism  of  chemical  reactions, 
it  was  pointed  out  that  the  removal  of  certain  products  from  the 
sphere  of  action  would  result  in  the  reaction  taking  a  certain 
course  according  to  the  principle  of  mass  action.  The  removal  of 
products  may  be  brought  about  by  mechanical  means  or  by 
chemical  reagents.  In  the  latter  case,  such  actions  must  not  be 
confused  with  catalytic  actions,  but  at  the  same  time,  the  actions 
of  catalysts  must  not  be  interpreted  as  being  of  such  a  nature. 
It  is  difficult  at  times  to  decide  whether  a  given  substance  is 
acting  solely  as  a  catalyst,  or  also  by  the  removal  of  certain 
products  of  reaction.  Thus,  the  decomposition  of  formic  acid  as 
formulated  in  equations  (2)  may  follow  (a)  in  the  presence  of 
sulfuric  acid  perhaps  because  of  the  combination  of  the  acid 
with  the  water,  and  (b)  perhaps  because  of  the  combination  of 
the  platinum,  rhodium,  ruthenium,  or  iridium,  with  the  hydro- 
gen, but  the  conclusions  are  not  certain  and  the  possibility  re- 
mains that  the  reactions  are  catalytic.  In  most  cases,  however, 
there  is  no  room  for  doubt  of  this  nature. 

The  phenomenon  of  negative  catalysis,  or  retardation  by 
catalysts,  has  been  shown  to  be  possible  if  the  velocity  of  the 
reaction  in  the  presence  of  the  catalyst  substance  is  less  than  in 
its  absence  with  certain  limitations  as  to  the  difference  in  these 
velocities  and  the  amount  of  catalyst  substance  as  already 
pointed  out.  Most  of  the  phenomena  which  have  been  termed 
negative  catalyses  are  due,  however,  to  combination  of  the  cata- 
lyst with  one  or  more  of  the  products  of  the  reaction,  or  to  the 
presence  of  so-called  catalyst  poisons  which  appear  to  act  by 
combining  with  the  catalyst  and  thus  preventing  its  action. 

The  action  of  a  catalyst  may  make  itself  apparent  by  caus- 
ing the  reaction  to  take  a  different  course,  as  already  stated  and 


THEORY  OF  CATALYTIC  ACTIONS  49 

as  indicated  in  equations  (7a)  and  (7c).  These  phenomena 
might  be  considered  in  the  most  general  way  to  involve  two 
possibilities.  The  reaction  between  two  or  more  products  tends 
to  form  an  addition  compound  which  may  react  further  to  form 
different  sets  of  products  (equations  (7b)  and  (7c) )  with  veloci- 
ties characteristic  for  each  set  under  the  given  conditions.  The 
participation  of  the  catalyst  substance  in  the  make-up  of  the 
addition  compound  and  the  subsequent  breaking  down  of  the 
latter  to  form  the  same  sets  of  products  as  in  the  first  case  plus 
catalyst  substance,  may  very  well,  because  of  the  presence  of 
the  catalyst  substance,  take  place  with  quite  different  velocities. 
There  may  be  a  greater  number  of  successive  reactions  in  the 
presence  of  the  catalyst.  A  different  possibility  of  the  action  of 
the  catalyst  substance  results  if  the  view  is  taken  that  because 
of  it  the  reactions  are  fundamentally  different;  that  a  different 
addition  compound  is  formed,  that  the  formation  as  well  as  the 
decomposition  of  the  addition  compound  will  be  different  because 
its  constituents  are  different.  Different  sets  of  products,  different 
velocities  for  the  same  sets  of  products  plus  catalyst,  etc.,  might 
well  be  expected  under  these  conditions.  An  example  of  this  has 
already  been  given.  In  equations  (1)  and  (4),  the  reaction 
between  alcohol  and  acid  in  the  absence  and  presence  of  a  cata- 
lyst would  give  the  same  set  of  products  as  shown  in  (a)  and  (b) 
for  both,  but  in  the  presence  of  the  catalyst  the  additional  pos- 
sibility (c)  is  present,  and  is  observed  to  a  considerable  extent 
if  a  tertiary  alcohol  is  used.  '  0 

The  explanation  of  catalytic  actions  on  the  basis  of  one  of 
the  two  preceding  views  is  greatly  a  matter  of  personal  inclina- 
tion at  present.  In  cases  where  sufficient  experimental  evidence 
is  available  there  is,  as  a  rule,  no  room  for  doubt.  In  other  cases 
the  matter  is  of  minor  importance  and  cannot  be  decided  satis- 
factorily unless  more  data  are  at  hand.  This  is  illustrated  by 
the  examples  which  follow,  in  which  different  products  are  ob- 
tained depending  upon  the  catalyst  substances  used. 

The  decomposition  of  formic  acid  as  shown  in  equations  (5) 
is  an  interesting  example.  A  recent  study  *  showed  that  formic 
acid  vapor  passed  through  a  platinum  tube  at  1150°  formed  in 
the  first  instance  hydrogen  and  carbon  dioxide.  These  products 

1  J.  A.  Muller  and  E.  Peytral,  Bull.  soc.  chim.  Z9,  34   (1921). 


50  CATALYTIC  ACTION 

in  some  cases  or  under  certain  conditions  reacted  farther  to  form 
water  and  carbon  monoxide,  or  the  hydrogen  reacted  with  formic 
acid  to  give  hydrocarbons,  but  the  primary  reaction  was  the 
simple  decomposition  indicated.  The  direct  formation  of  carbon 
monoxide  and  water  by  heating  formic  acid  with  sulfuric  acid 
(and  presumably  with  other  mineral  acids)  was  considered  by 
these  investigators  to  involve  several  steps,  and  not  to  be  only  a 
dehydrating  action. 

The  decomposition  of  alcohol  may  give  different  products  as 
follows:  — 

f  1   =  2C2H4  +  2H20  (a) 

2C2H5OH      =  2CH3CHO  +  2H2  (b)  (8) 

L  J  =  (C2H5)20  +  H20  (c) 

The  reaction  may  follow  equation  (a)  or  equation  (b),  or  both,  in 
the  presence  of  different  oxides;  with  thoria,very  little  of  the  reac- 
tion products  shown  in  (b)  are  formed,  with  magnesia,  very  little 
of  those  shown  in  (a),  while  other  oxides  give  results  intermediate 
between  these.1  These  reactions  will  be  taken  up  again  in  Chap- 
ter VIII.  The  substances  shown  in  equations  (a)  and  (c)  may 
be  obtained  by  heating  the  alcohol  with  sulfuric  acid;  ethylene 
being  formed  preferably  at  one  temperature,  ether  at  another. 
Ether  has  also  been  obtained  by  heating  the  alcohol  at  higher 
temperatures  with  oxides  such  as  alumina. 

Ethyl  acetacetate  may  decompose  according  to  the  following 
equations:  — 


f 

[_ 


rn  corn  ro  r  H  1=  CH3COCHa  +  C02  +  C2H5OH  (a) 

CH0     HC0CH 


Reaction  (a),  ketone  decomposition,  takes  place  on  heating  with 
dilute  sulfuric  acid  or  with  dilute  aqueous  solution  of  alkali  ;  re- 
action (b),  acid  decomposition,  by  heating  with  a  concentrated 
solution  of  alcoholic  alkali.  Whether  these  reactions  are  to  be 
classed  as  catalytic  will  depend  upon  the  definition  of  catalysis 
which  is  adopted. 

Chlorine  and  benzene  react  in  sunlight  to  form  addition  com- 

JP.  Sabatier  apd  A.  Mailhe,  Ann.  Chim.  Phys.  (8)  %0,  28$  (1910), 


THEORY  OF  CATALYTIC  ACTIONS  51 

pounds ;  in  the  presence  of  iodine  chloride  both  addition  and  sub- 
stitution products  are  formed;  and  with  stannic  chloride  or  ferric 
chloride  only  the  substitution  products  are  obtained.1 

With  hydrazine,  the  following  reactions  have  been  found  to 
occur: — 2 


6N2H4      = 


2N2  (a) 

6NH3  +  3N2  +  3H2  (b)  (10) 

4NH3  +  4N2  +  6H2  (c) 


Reaction  (a)  mainly  occurs  with  hydrazine  sulfate,  reaction  (b) 
with  free  hydrazine,  and  reaction  (c)  in  the  presence  of  alkali. 

Many  examples  of  oxidation  and  reduction  of  organic  and 
also  of  inorganic  compounds  might  be  mentioned  in  this  connec- 
tion. The  possibility  of  obtaining  different  products  rests  fre- 
quently upon  the  special  reagent  which  is  employed.  It  will 
hardly  be  necessary  to  give  specific  examples  of  such  reactions. 
A  number  of  them  belong  to  the  group  of  catalytic  reactions. 
The  use  of  various  metals  in  many  oxidations  or  reductions  need 
only  be  mentioned. 

A  different  group  of  reactions  is  included  in  the  actions 
of  different  enzymes  upon  the  same  material  giving  different 
products.  An  interesting  example  which  may  be  quoted  involves 
the  decomposition  of  the  trisaccharide  raffinose.3  Sucrase  (top 
yeast  extract)  hydrolyzed  raffinose  to  form  melibiose  and  fruc- 
tose, while  emulsin  hydrolyzed  raffinose  to  form  sucrose  and 
galactose. 

These  examples  will  serve  to  show  the  nature  of  these  reac- 
tions and  may  also  indicate  problems  which  might  profitably  be 
studied. 

The  changes  in  reaction,  either  in  the  velocity  or  in  the  course, 
brought  about  by  catalyst  substances  may  be  looked  upon  from  a 
different  point  of  view  as  changes  in  the  environment  which 
manifest  themselves  in  the  indicated  manner.  The  catalyst  sub- 
stances bring  about  these  changes  in  reaction  in  a  definite  way, 
one  step  being  the  formation  of  addition  products.  Changes  in 
reaction  can  be  brought  about  also  by  altering  the  physical  con- 

»A.  Slator,  J.  Chem.  Soc.  83,  729   (1903). 

»S.  Tanatar,  Z.  physik.  Chem.  W,  475;  41,  37   (1902). 

•C,  S,  Hudson,  Jour.  Amer,  Chem,  Soc.  36,  1566   (1914), 


52  CATALYTIC  ACTION 

ditions,  by  removing  products  of  reaction,  etc.  Catalyst  actions 
form  therefore  one  of  the  ways  in  which  different  reactions  may 
occur  starting  with  the  same  substances.  It  is  of  interest  to  note 
that  Berzelius  looked  upon  catalysis  from  this  point  of  view  as 
shown  by  the  quotation  from  his  writings  given  on  page  12  in 
considering  changes  in  the  body  where  from  the  same  liquid 
(blood)  in  different  parts  different  products  may  be  formed. 

Following  directly  from  the  considerations  given,  it  is  clear 
that  very  small  amounts  of  substances  can  act  as  catalysts  since, 
the  catalyst  substance  is  formed  again  in  the  decomposition  of 
the  addition  compound  of  which  it  forms  a  part.  This  fact  has 
been  used  in  the  past  as  one  of  the  criteria  of  catalytic  actions, 
and  it  is  also  a  direct  outgrowth  or  development  of  the  views 
outlined  here.  At  the  same  time,  the  definition  of  catalyst  is 
not  limited  to  small  amounts  of  substances  according  to  these 
views.  The  amount  does  not  come  into  question  as  far  as  the 
theoretical  significance  of  the  views  is  concerned.  It  may  vary 
from  infinitesimal  amounts  up  to  being  the  predominating  sub- 
stance present.  The  latter  would  be  the  case  with  many  reac- 
tions taking  place  in  aqueous  solutions  where  the  water  would 
be  considered  to  be  the  catalyst.  At  the  same  time,  all  of  the 
water  present  would  not  be  acting  as  catalyst  all  the  time,  but 
only  that  portion  actually  combined  with  the  ions  or  molecules 
which  are  reacting. 

This  raises  another  question  with  regard  to  the  action  of  the 
catalyst  in  considering,  for  example,  possible  changes  in  reaction 
velocities.  With  very  small  concentrations  of  catalyst  substances 
and  large  amounts  of  other  reacting  materials,  assuming  no  sec- 
ondary reactions  of  the  products  with  catalyst,  etc.,  a  definite 
fraction  of  the  catalyst  present  would  continually  exist  in  com- 
bination in  the  addition  compound  and  the  remainder  (possibly 
only  a  very  small  amount)  would  be  present  free  in  the  reaction 
mixture.  A  steady  state  would  exist,  and  under  these  conditions 
a  definite  constant  change  (increase)  in  velocity  due  to  the  cata- 
lyst would  occur.  With  increasing  quantities  of  catalyst,  after 
a  certain  stage  this  steady  state  would  no  longer  be  probable,  but 
the  concentration  of  combined  catalyst  would  change  as  the  re- 
action proceeded,  depending  upon  the  relative  concentrations  of 
reacting  substances  and  catalyst  substance,  This  condition  would 


THEORY  OF  CATALYTIC  ACTIONS  53 

also  occur  with  small  amounts  of  catalyst  after  a  great  part  of 
the  initial  substances  had  been  transformed.  Under  these  con- 
ditions the  change  in  velocity  due  to  the  presence  of  the  cata- 
lyst will  not  be  constant  but  will  change  as  the  reaction  pro- 
ceeds because  of  the  changes  in  relative  concentrations  of  the 
reacting  substances,  including  the  catalyst. 

The  theory  of  catalytic  actions  as  based  upon  the  single  cri- 
terion of  one  substance  (identified  as  the  catalyst)  in  a  chemical 
reaction  possessing  the  same  composition  after  the  reaction  as 
before  appears  to  be  straightforward  and  direct  and  simple  of 
application.  It  has  been  shown  how  a  number  of  the  phenomena 
ordinarily  grouped  under  catalysis  follow  by  the  application  of 
simple  chemical  principles,  and  the  necessary  limitations  of  the 
deductions  which  develop  directly  from  these  applications  have 
also  been  pointed  out.  Further  relations  of  similar  nature  will 
be  presented  in  the  succeeding  chapters. 

The  remainder  of  this  chapter  will  be  devoted  to  the  consid- 
eration of  a  difficulty  encountered  in  the  application  of  the  views 
as  outlined.  The  significance  of  the  views  is  simple  and  not  the 
cause  of  the  difficulty;  it  is  in  the  study  of  certain  reactions 
which  from  one  point  of  view  are  definitely  catalytic  but  which 
do  not  conform  to  the  strict  definition  as  given.  The  funda- 
mental difficulty  lies  in  the  fact  that  it  is  difficult  to  limit  a  re- 
action to  one  definite  change.  The  products  of  a  reaction  may 
react  farther,  the  catalyst  substance  from  its  very  nature  being 
far  from  inert,  may  participate  in  reactions  with  the  products 
and  cause  complications,  etc.  In  other  words,  in  the  experi- 
mental study  of  the  reactions,  the  meaning  to  be  ascribed  to  the 
words  "same  composition  after  the  reaction  as  before"  forms 
the  crux  of  the  difficulty.  In  the  hydrolysis  of  sucrose  or  of 
esters  by  acid,  in  the  combination  of  hydrogen  and  oxygen  in 
the  presence  of  platinum  or  palladium  black,  in  the  combination 
of  ammonia  and  hydrogen  chloride  in  the  presence  of  water 
vapor,  etc.,  this  question  does  not  arise.  In  the  Friedel-Crafts 
reaction,  the  reaction  is  carried  out  experimentally  in  several 
stages  in  one  of  which  the  aluminium  chloride,  the  catalyst,  is 
decomposed  intentionally.  Here,  in  the  classification,  only  the 
first  part  of  the  reaction  would  be  considered  catalytic,  but  even 
here  the  possibility  exists  that  the  aluminium  chloride  is  com- 


54  CATALYTIC  ACTION 

bined  with  a  product  of  the  reaction.  In  the  hydrolysis  of  an 
ester  by  a  base  such  as  sodium  hydroxide,  according  to  the  equa- 
tion 

RC02R'  +  Na+  +  OH-  =  RC(V  +  Na+  +  R'OH,       (11) 

omitting  the  part  played  by  the  solvent  water,  there  is  no  reason 
to  consider  the  hydroxyl  ion  as  the  catalyst.  The  Na+  ion,  ac- 
cording to  equation  (11)  would  be  the  catalyst.  This  is  contrary 
to  experiment,  however,  since  it  is  found  that  the  reaction  is  due 
to  the  OH~  ion  and  not  to  the  Na+  ion.  The  more  nearly  correct 
equation  would  be 

RC02R'  +  OH'  =  RC02-  +  R'OH  ( 12) 

omitting  the  possible  action  of  the  solvent  again.  This  reaction 
would  therefore,  strictly  speaking,  not  be  classed  as  catalytic. 

The  catalytic  decomposition  of  imido  esters  as  studied  by  J. 
Stieglitz  and  his  co-workers  to  which  extended  reference  was 
made  in  the  last  chapter,  may  be  referred  to  again  in  this  connec- 
tion. The  reaction  velocity  is  increased  by  the  addition  of  acid 
(hydrochloric  acid,  for  example)  but  in  the  final  products  am- 
monium chloride  is  present.  The  question  as  to  the  exact  cata- 
lyst is  pertinent.  Stieglitz  showed  that  the  increased  velocity  was 
due  to  the  increase  in  concentration  of  the  constituents  (ions 
or  molecules)  which  reacted.  Hydrochloric  acid  increased  the 
concentration  of  the  reacting  ion  by  forming  a  highly  ionized 
salt.  Hydrochloric  acid  was  added  as  such  but  in  the  reaction 
mixture  was  present  combined  in  the  imido  ester  salt.  The  re- 
action studied  took  place  in  a  number  of  stages  which  were  in- 
vestigated separately.  According  to  the  definition  used  here  and 
interpreted  strictly,  hydrochloric  acid  would  not  be  considered 
the  catalyst  in  this  reaction. 

This  raises  at  once  another  pertinent  question.  If  the  care- 
ful study  of  such  a  reaction  as  the  imido  ester  hydrolysis  has 
interpreted  in  a  satisfactory  way  the  mechanism  of  this  reac- 
tion without  introducing  conceptions  not  already  included  in 
chemical  theory,  is  there  any  reason  to  class  this  reaction  as 
catalytic.  From  the  point  of  view  developed  here,  there  would 
be  none.  Stieglitz  considered  change  in  reaction  velocity  as  the 


THEORY  OF  CATALYTIC  ACTIONS  55 

only  characteristic  property  of  catalytic  reactions,  but  as  shown 
in  Chapter  II,  this  definition  is  not  satisfactory  for  a  general 
classification.  The  conclusion  seems  to  be  unavoidable  that  a 
careful  study  of  many  reactions  classed  as  catalytic  at  the  present 
time,  would  make  it  possible  to  account  for  the  mechanisms  of 
these  reactions  in  definite  chemical  ways.  The  question  of  un- 
changed composition  of  a  catalyst  may  perhaps  be  best  answered 
by  considering  the  catalyst  substance  as  a  component  of  the 
reaction  mixture  in  the  sense  that  component  is  used  in  phase 
rule  discussions.  There  is,  however,  a  real  difficulty  here  which 
may  be  solved  by  the  introduction  of  a  suitable  definition  for 
"unchanged  composition."  Perhaps  the  further  consideration  and 
treatment  of  catalytic  reactions  from  the  point  of  view  advocated 
here  will  bring  defmiteness  to  the  relations  and  show  the  most 
satisfactory  meaning  of  the  term  to  be  adopted. 


Chapter  IV. 
Energy  Relations. 

In  the  treatment  of  catalytic  actions,  a  change  in  reaction 
velocity  has  generally  been  looked  upon  as  one  of  the  most  strik- 
ing and  characteristic  features  of  such  actions  and  has  frequently 
been  used  as  the  chief  criterion.  It  was  pointed  out  in  Chapter 
II  that  a  change  in  reaction  velocity  as  the  basic  definition  of 
catalysis  was  not  altogether  satisfactory.  In  Chapter  III  a  dif- 
ferent definition  of  catalysis  was  outlined.  It  was  shown  that 
this  latter  view  brought  out  the  reasons  for  the  significance  and 
importance  ascribed  to  changes  in  reaction  velocity  with  cata- 
lytic reactions,  or  perhaps  better,  with  reactions  which  were 
grouped  under  the  term  catalytic  by  more  or  less  common  con- 
sent. Change  in  reaction  velocity  is  a  definite  sign  or  symptom 
of  catalytic  change.  It  is  a  very  important  sign,  but  not  the  only 
one.  It  is  secondary  to  the  more  fundamental  phenomenon 
which  really  characterizes  catalytic  actions  and  which  was  dis- 
cussed in  some  detail  in  the  preceding  chapter. 

The  consideration  of  the  mechanism  of  a  chemical  reaction, 
including  its  velocity,  raises  the  question  of  the  reaction  in  the 
reverse  direction.  All  chemical  reactions  are  reversible  theoreti- 
cally, many  are  so  practically.  The  reversibility  of  a  chemical 
reaction  and  its  connection  with  reaction  velocity  leads  directly 
to  the  concept  of  equilibrium  conditions  of  the  reaction.  This 
can  be  shown  readily  by  means  of  the  following  equations  : 

Y!  =  Iq^C,  ......          V2  =  kjC'^V  •  •  •  (1) 

Vi  =  v2  (2) 

Then                              ki^cV/,...  (3) 
k2 


56 


ENERGY  RELATIONS  57 

Denoting  -r^by  K,  then 

-  =  K  (4) 

At  equilibrium,  the  velocity  of  the  reaction  in  one  direction 
is  equal  to  the  velocity  in  the  opposite  direction  (equation  (2)  ). 
Under  this  condition  the  composition  of  the  mixture  would 
remain  unchanged.  The  concentrations  of  the  various  sub- 
stances present  would  then  be  given  by  equations  (3)  or  (4). 
The  constant  K  is  the  well-known  equilibrium  constant  whose 
value  is  determined  by  the  mass  action  law.  As  deduced,  K  is 
shown  to  be  constant  at  constant  temperature  and  pressure.  The 
limitations  inherent  in  the  velocity  equations  are  to  be  found  in 
the  equilibrium  expression  as  well.  Thus,  the  c  terms  represent 
concentrations,  and  the  equation  is  really  an  expression  of  the 
law  of  concentration  action,  and  not  directly  of  mass  action,  as 
already  indicated  in  Chapter  II.  The  equations  as  deduced  are 
true  only  for  dilute  solutions  and  for  gases  at  not  too  great  pres- 
sures. 

The  equation  for  the  equilibrium  constant  can  also  be  de- 
duced on  purely  thermodynamic  considerations.  This  is  done 
with  the  aid  of  the  conception  of  the  equilibrium  box  (van't 
Hoff ) .  This  deduction  involves  the  assumption  of  the  validity  of 
the  simple  gas  laws  and  is  therefore  also  limited  to  dilute  solu- 
tions or  to  gases  at  moderate  pressures. 

In  the  development  of  the  thermodynamic  treatment  of  chem- 
ical reactions,  the  equilibrium  constants  of  chemical  mixtures  are 
connected  with  certain  energy  relationships.  Thermodynamics 
does  not  involve  a  time  function  and  consequently  the  theoreti- 
cal developments  based  upon  thermodynamics  alone  cannot  di- 
rectly involve  such  a  relation  as  reaction  velocity.  As  will  be 
shown  in  Chapter  V,  it  is  necessary  to  bring  in  some  different 
viewpoint  such  as  the  treatment  based  upon  statistical  mechanics 
to  include  the  conception  of  time  as  required  in  the  reaction 
velocity  relationships.  Catalysis  has  frequently  been  assumed 
heretofore  to  involve  a  change  in  reaction  velocity,  and  thermo- 
dynamics is  not  applicable  directly  to  reaction  velocities.  There 
does  not  therefore  appear  to  be  any  point  of  direct  contact  be- 


58  CATALYTIC  ACTION 

tween  catalysis  and  thermodynamics  at  first  sight.  However, 
some  relations  have  been  developed  in  the  past,  which,  although 
in  a  sense  negative  in  character,  are  of  fundamental  interest. 
Also,  it  is  possible  that  if  a  more  general  or  less  limited  defini- 
tion of  catalysis  is  accepted,  further  applications  of  thermody- 
namics to  catalytic  reactions  may  be  expected. 

The  first  question  which  may  be  taken  up  is  the  influence  of 
the  catalyst  on  the  equilibrium  constant,  and,  since  the  equilib- 
rium constant  may  be  considered  to  be  made  up  of  the  velocity 
constants  of  the  two  opposing  reactions,  on  these  velocity  con- 
stants, or  on  the  velocities  of  these  reactions.  The  discussions 
in  the  past  have  been  centered  mainly  upon  certain  definitions 
which  have  been  proposed  and  from  which  definite  deductions 
were  made.  The  first  important  property  of  a  catalyst  which 
has  been  accepted  widely  is  that  it  increases  the  velocity  of  a 
chemical  reaction,  the  second  that  it  is  itself  unchanged  as  a 
result  of  the  reaction.  Now,  van't  Hoff  showed  that  if  the  cata- 
lyst is  unchanged  as  a  result  of  the  reaction,  the  equilibrium  of 
the  reaction  must  be  the  same,  whether  or  not  the  catalyst  is 
present,  since  otherwise  energy  could  be  obtained  from  the  reac- 
tion without  introducing  it  in  any  way.  Analyzing  this  view  a 
little  more  carefully,  it  is  evident  that  the  crux  of  the  question  is 
to  be  found  in  the  significance  of  the  statement  that  the  catalyst 
is  unchanged  in  the  reaction.  It  is  the  same  question  which  was 
discussed  at  the  end  of  Chapter  III  in  considering  the  definition 
of  catalytic  actions.  From  the  point  of  view  of  the  equilibrium 
expression  (equation  (4)  ),  it  may  be  taken  to  signify  that  the 
concentration  term  c  representing  the  catalyst  is  the  same  in  the 
numerator  and  in  the  denominator  of  the  fraction,  or  the  same  for 
the  initial  and  the  final  states  of  the  reaction  mixture.  Under 
these  conditions  the  value  of  K  will  be  the  same  in  the  presence 
and  absence  of  the  catalyst,  and  the  change  in  freer  energy  (oj> 
the  chemical  affinity)  of  the  reaction  will  be  identicar^hEttTeror 
not  the  catalyst  is  present. 

This  conclusion  is  definite  and  clean-cut,  but  in  the  applica- 
tion to  chemical  reactions,  it  is  soon  evident  that  it  represents  a 
set  of  conditions  limited  to  a  more  or  less  ideal  case.  The  chemi- 
cal equation  representing  a  chemical  reaction  is,  as  a  rule,  defi- 
nite in  that  a  specific  change  or  reaction  is  illustrated.  The 


ENERGY  RELATIONS  59 

mathematical  equations  involve  deductions  from  the  chemical 
equation  by  the  application  of  mathematical  processes  to  terms 
which  represent  to  a  greater  or  less  degree  of  approximation  the 
chemical  terms  or  substances  in  the  chemical  equation.  In  these 
equations,  first  chemical  and  then  mathematical,  the  changes 
considered  are  perfectly  definite.  In  the  actual  chemical  reac- 
tion, the  conditions  are  hardly  ever  such  that  the  chemical  and 
mathematical  equations  denote  the  complete  condition  of  affairs. 
Generally  more  than  one  change  is  possible  in  the  chemical  re- 
action and  the  chemical  equation  may  represent  only  the  change 
in  which  the  worker  is  interested.  Other  changes  which  may 
occur  are  ignored.  Also,  substances  which  take  part,  but  whose 
final  composition  is  the  same  as  their  initial  composition,  and 
which  apparently  are  not  involved,  are  not,  as  a  rule,  included  in 
the  chemical  equation.  Solvents  and  their  actions  are  frequently 
omitted.  From  the  definition  of  catalysts  used  here,  it  can  be 
seen  that  they  also  would  frequently  not  be  included  in  the 
chemical  equations. 

The  three  concepts,  chemical  reaction,  chemical  equation,  and 
mathematical  equation,  are  supposed  to  describe  the  same  phe- 
nomenon in  any  given  case.  Actually  they  do  so  only  as  an  ideal 
condition,  and  the  possibility  of  deviation  becomes  greater  with 
increasing  complexity  of  the  reactions  and  with  decreasing  care 
in  the  use  of  terms  and  expressions.  To  develop  these  views 
somewhat  farther  it  is  evident  that  the  composition  of  a  chemical 
constituent  in  a  reaction  mixture  might  be  unchanged  as  the  re- 
sult of  a  reaction  but  that  it  might  be  in  combination  with  one 
or  another  constituent  in  that  mixture.  For  example,  the  action 
of  an  acid  such  as  hydrochloric  acid  in  aqueous  solution  hydro- 
lyzing  an  ester  such  as  methyl  acetate,  would  as  a  rule  be  classed 
as  catalytic.  The  question  then  comes  up,  if  the  catalyst  is  to  be 
represented  in  the  mathematical  expression  (equation  (1)  )  what 
concentration  should  be  used.  For  a  time  the  concentration  of 
the  molecular  hydrogen  chloride  (HC1)  was  used.  More  recently 
the  hydrogen  ion  (H+)  has  been  assumed  to  be  the  catalyst. 
However  it  is  known  that  the  hydrogen  ion  in  aqueous  solution 
is  hydrated  and  the  term  representing  the  concentration  of  this 
particular  molecular  species  should  have  the  form  (H.(H20)X)+. 
In  the  presence  of  an  ester  the  matter  is  complicated  still  farther 


60  CATALYTIC  ACTION 

as  the  hydrogen  ion  may  also  be  in  combination  with  it  to  a  cer- 
tain extent.  After  the  hydrolysis  reaction  has  gone  on  for  a  time, 
the  presence  of  alcohol  and  organic  acid  with  their  possibilities 
of  combination  with  the  hydrogen  ion  (or  possibly  hydrogen 
chloride  molecule)  adds  to  the  complications.  It  would  then  be 
difficult  to  determine  the  concentration  term  in  the  mathematical 
expression  which  would  contain  the  hydrogen  chloride  or  hydro- 
gen ion.  Probably  several  would  contain  it.  It  follows  there- 
fore that  whether  the  hydrogen  and  chlorine  would  still  be  con- 
sidered to  be  associated  with  each  other  as  hydrogen  chloride, 
or,  in  aqueous  solution  as  hydrochloric  acid,  and  producing  hy- 
drogen ions  in  the  latter  case,  the  use  of  a  simple  concentration 
term  in  the  mathematical  equation  to  represent  this  substance 
is  open  to  question.  Similar  relations  hold  for  practically  all 
catalytic  reactions  taking  place  in  aqueous  solutions  and  prob- 
ably for  many  reactions  in  non-aqueous  solvents.  For  reactions 
in  gaseous  systems,  similar  relations  hold.  In  the  catalysis  of 
the  reaction  between  ammonia  and  hydrogen  chloride  by  water 
vapor,  it  can  readily  be  seen  that  the  water  vapor  may,  to  an 
increasingly  greater  degree,  be  combined  with  the  ammonium 
chloride  formed,  as  the  reaction  proceeds.  In  the  reactions  in- 
cluded under  contact  catalysis,  which  will  be  considered  more 
in  detail  in  Chapter  VIII,  the  relations  do  not  appear  to  be  so 
simple,  but  if  each  reaction  of  this  type  is  considered  independ- 
ently, it  will  be  possible  to  determine  to  what  extent,  if  any, 
the  state  of  combination  of  the  catalyst  has  changed  during  the 
reaction. 

A  comparison  of  the  chemical  reaction,  chemical  equation, 
and  mathematical  equation,  will  throw  light  on  many  of  the  ques- 
tionable points  involved.  Each  of  these  concepts  has  been  evolved 
on  certain  definite  bases,  and  each  possesses  certain  limitations. 
The  treatment  of  successive  reactions  also  serves  to  illustrate  this 
question.  The  use  of  a  chemical  equation  to  represent  one  or 
more  of  the  steps  of  a  series  of  successive  reactions  is  greatly  a 
matter  of  choice  or  convenience  and  the  mathematical  equation 
will  then,  in  addition  to  the  limitations  inherent  in  its  nature,  also 
contain  the  limitations  included  in  the  application  of  the  chemi- 
cal equation  to  the  chemical  reaction.  The  chemical  and  mathe- 
matical equations  may  then  furnish  only  an  approximate  and 


ENERGY  RELATIONS  61 

one-sided  picture  of  the  chemical  reaction  which  is  being  con- 
sidered. These  views,  applicable  to  chemical  reactions  in  gen- 
eral, will  bring  out  the  weaknesses  of  the  classification  of  a  group 
of  reactions  such  as  catalytic  reactions.  The  difficulty  of  defin- 
ing in  a  completely  satisfactory  manner  the  properties  of  such  a 
group  of  reactions  is  striking.  Reactions  which  may  or  may  not 
be  assigned  to  the  group  will  be  found  as  soon  as  a  definition, 
more  or  less  rigid,  is  suggested.  This,  again,  raises  the  question 
as  to  the  permanent  value  of  a  grouping  such  as  that  involved 
in  catalysis.  It  may  be  repeated,  without  making  a  definite 
statement  as  to  its  permanent  value  at  the  present  time,  that 
the  grouping  has  been  of  value  in  calling  attention  to  the  un- 
known mechanism  of  these  reactions  and  the  probable  relations 
to  reactions  in  general. 

To  return  to  the  question  of  state  of  combination  of  the 
catalyst,  it  is  seen  that  in  catalytic  reactions,  even  if  the  cata- 
lyst is  apparently  unchanged,  it  will  not  always  be  possible  to 
use  the  same  concentration  term  for  it  in  the  concentration  law 
equation  for  the  initial  and  final  products  of  the  reaction.  The 
chemical  equation  as  ordinarily  written  is  incomplete  in  that  it 
does  not  show  the  action  of  the  solvent  for  reactions  taking  place 
in  solutions,  the  action  of  so-called  "adsorption"  for  reactions 
taking  place  in  two  phased  systems,  etc.  The  general  conclu- 
sion arrived  at  then  is  that,  unless  the  catalyst  is  entirely  un- 
combined,  or  is  combined  in  the  same  way  before  and  after  the 
reaction  has  taken  place,  the  equilibrium  constant  of  the  reaction 
may  not  be  the  same  in  the  presence  and  absence  of  the  cata- 
lyst. In  the  ideal  limiting  case  of  no  combination  or  combina- 
tion to  the  same  extent,  the  equilibrium  of  the  reaction  will  not 
be  changed  by  the  presence  of  the  catalyst  substance. 

The  same  conclusion  has  been  arrived  at  by  a  number  of 
workers  by  considering  energy  relationships  of  catalytic  reac- 
tions. Among  these  may  be  quoted  G.  Bredig,1  who  stated  that 
if  a  catalyst  is  changed  in  a  physical  or  chemical  manner  in  the 
catalyzed  reaction,  the  equilibrium  of  the  reaction  would  not  be 
independent  of  the  catalyst,  but  would  be  dependent  upon  its 
nature  and  amount.  A  change  in  the  vapor  pressure  of  the 
catalyst  necessitates  a  difference  in  the  work  required  to  remove 

1  Ergebnisse  der  Physiolopie  1,  139   (}902), 


62  CATALYTIC  ACTION 

the  catalyst  from  the  reaction  mixture.  As  long  as  the  amount 
of  this  work  is  the  same  under  the  same  conditions  before  and 
after  the  reaction,  the  equilibrium  would  remain  unchanged.  If 
the  catalyst  is  present  in  large  excess  it  would  act  as  solvent.  A 
change  in  the  nature  of  the  solvent  changes  the  equilibrium  and 
only  in  dilute  solution  would  the  equilibrium  here  remain  the 
same.  These  statements  were  repeated  in  somewhat  different 
forms  by  various  workers  and  some  further  points  added  and 
clarified  without  being  adopted,  however,  in  the  general  litera- 
ture of  catalytic  actions.1  Among  those  who  have  brought  out 
views  similar  to  those  of  Bredig  may  be  mentioned  E.  Abel,2  who 
stated,  assuming  the  formation  of  intermediate  products  with 
the  catalyst,  that  if  the  catalyst  was  in  a  different  chemical  or 
physical  state  at  the  end  of  the  reaction  from  what  it  was  at  the 
beginning,  it  had  given  up  or  received  energy,  and  that  conse- 
quently a  change  in  the  equilibrium  was  conceivable;  J.  Stieg- 
litz,3  who  pointed  out  the  conditions  under  which  the  equilibrium 
of  a  catalyzed  reaction  would  be  changed;  M.  A.  Rosanoff,4  who 
spoke  of  the  possibility  of  a  catalyst  influencing  the  equilibrium, 
and  that  it  did  not  do  so  only  when  the  molecular  state  of  the 
reagents  was  not  affected  by  the  catalyst;  W.  D.  Bancroft,5  and  a 
number  of  others. 

It  would  be  possible  to  describe  a  number  of  catalytic  reac- 
tions in  which  the  equilibrium  has  been  changed  as  well  as  not 
changed  by  the  catalyst.  It  would  lead  too  far,  however,  to  do 
this  in  the  present  connection.  Reference  to  the  larger  text  books 
and  compilations  of  catalytic  reactions  must  suffice.  The  main 
point  is  that  the  equilibrium  may  be  changed  by  the  presence  of 
the  catalyst  if  the  views  regarding  catalysis  as  developed  here 
are  adopted.  If,  by  definition,  a  catalyst  can  not  change  the 
equilibrium  of  a  reaction,  then  a  discussion  of  the  question  is 
unnecessary. 

The  application  of  these  considerations  to  the  velocities  of 
catalytic  reactions  is  simple  in  principle.  As  stated,  the  equilib- 
rium constant  is  taken  to  be  made  up  of  the  two  velocity  con- 

1Cf.  for  example  E.  K.  Rideal  and  H.  S.  Taylor,  "Catalysis  in  Theory  and 
Practice,"  1919,  pp.  18-23. 

2Z.  Elektrochem.  13,  555  (1907). 

3  Am.  Chem.  J.  39,  56  (1908). 

*Jour.  Amer.  Chem.  Soc.  35,  17.°>  (191-3). 

8  Jour.  Physic.  Chem.  21,  573  (1917), 


ENERGY  RELATIONS  63 

slants  of  the  opposing  reactions.  If  the  equilibrium  of  a  reac- 
tion is  unchanged  by  the  addition  of  a  catalyst,  and  if  the  cata- 
lyst increases  the  velocity  of  the  reaction  in  one  direction,  then 
it  must  to  a  corresponding  degree,  act  as  a  catalyst  for  the  op- 
posing reaction  and  increase  its  velocity.  This  deduction  was 
made  a  number  of  years  ago,  and  the  general  qualitative  state- 
ment has  been  found  to  be  true,  although  quantitative  results 
to  test  this  point  are  not  at  hand  in  sufficient  amount  for  satis- 
factory conclusions  to  be  reached.  If,  however,  the  equilibrium 
of  a  reaction  is  changed  by  the  presence  of  a  catalyst,  then  the 
velocities  of  the  opposing  reactions  need  not  be  affected  by  the 
catalyst  to  the  same  extent,  and  it  is  readily  conceivable  that  the 
velocity  of  a  reaction  in  one  direction  might  be  increased,  while 
that  of  the  reaction  in  the  opposite  direction  not  be  affected  at 
all.  This  may  be  tantamount  to  saying  that  the  catalyst  for  the 
one  reaction  did  not  act  as  a  catalyst  for  the  opposing  reaction, 
and  from  the  preceding  discussion  might  be  explained  chemically 
by  considering  that  the  catalyst  substance  was  combined  with 
some  substance  in  the  reaction  mixture  which  prevented  it  from 
taking  part  in  the  desired  reaction. 

This  view  brings  with  it  a  chemical  significance  of  catalytic 
reactions  (as  well  as  of  other  phenomena)  which  has  been  men- 
tioned several  times,  but  whose  importance  cannot  be  over-em- 
phasized. The  point  which  it  is  desired  to  make  again  in  this 
connection  is  the  importance  which  certain  definitions  may  ac- 
quire, and  which  sometimes  obscure  the  relations  which  are  being 
studied.  If  these  definitions  are  accepted,  certain  conclusions 
follow  (such  as  the  velocities  of  opposing  reactions  being  in- 
creased by  the  catalyst  if  the  equilibrium  of  the  reaction  is  un- 
changed by  it) .  On  the  basis  of  some  other  definition,  different 
conclusions  might  be  obtained,  and  in  making  positive  statements 
with  regard  to  the  behavior  of  catalysts,  it  is  always  advisable 
to  examine  the  postulates  carefully. 

The  criteria  of  catalytic  actions  which  were  gradually  evolved 
in  the  course  of  a  number  of  years  do  not  appear  to  the  writer  to 
be  altogether  satisfactory.  It  is  for  this  reason  that  an  attempt 
is  being  made  to  formulate  a  simpler  definition,  including,  as  far 
as  possible,  the  relations  heretofore  developed,  at  the  same  time 


64  CATALYTIC  ACTION 

showing  the  bearing  on  chemical  phenomena  not  included  in 
catalysis. 

Another  point  may  be  spoken  of  in  this  connection.  The 
question  is  often  raised  whether  a  catalyst  can  start  a  reaction 
or  can  only  modify  its  velocity.  This  again  can  be  looked  upon 
as  a  matter  of  definition.  If,  by  definition,  a  catalyst  is  a  sub- 
stance which  changes  the  velocity  of  a  reaction,  then  there  seems 
to  be  no  reason  for  considering  that  it  can  start  a  reaction,  espe- 
cially if,  as  is  frequently  the  case,  the  proviso  is  added  that  it 
cannot  change  the  equilibrium.  If  the  definition  states  that  the 
catalyst  takes  part  and  is  unchanged  in  composition  after  the 
reaction,  then,  if  the  reaction  products  are  the  same  in  the  ab- 
sence and  in  the  presence  of  the  catalyst,  the  catalyst  does  not 
start  the  reaction.  If,  however,  the  products  are  different  in 
part  or  altogether  in  the  presence  of  the  catalyst,  then  the  re- 
action has  evidently  taken  a  different  course  to  some  extent,  at 
any  rate,  due  to  the  presence  of  the  catalyst,  and  in  this  sense 
the  catalyst  has  started  the  reaction.  .The  question  is  an  aca- 
demic one,  only  included  for  the  sake  of  completeness,  and  can 
be  answered  readily  in  any  given  case  by  stating  the  exact  defi- 
nition of  catalysis  which  is  being  used. 

In  the  discussion  so  far,  the  different  theories  of  chemical 
action  which  have  been  suggested  or  used  with  catalytic  reac- 
tions have  not  been  taken  up  in  detail.  For  instance,  the  cata- 
lytic actions  of  hydrogen  ions  or  acids  in  aqueous  solutions  on 
the  hydrolysis  of  sucrose  or  of  esters  has  not  been  discussed  or 
the  different  theories,  such  as  the  hydrogen  ion  view,  the  dual 
catalysis  view,  and  the  addition  compound  view,  given.  These 
phenomena  represent  more  or  less  special  cases,  and  since  they 
have  been  treated  by  a  number  of  workers  from  various  points  of 
view,  will  not  be  considered  farther  here.  Reference  only  will 
be  made  to  a  brief  discussion  by  the  writer  *  of  the  views  in 
connection  with  such  actions  of  acids  in  aqueous  solutions. 

Catalytic  reactions  form  a  group  of  chemical  reactions  dis- 
tinguished from  other  reactions  by  certain  more  or  less  marked 
properties  or  peculiarities.  The  exact  nature  of  these  properties 
is  to  a  great  extent  a  matter  of  definition,  and  many  of  the  ap- 
parent contradictions  and  exceptions  found  in  the  various  treat- 

1  "The  Chemistry  of  Enzyme  Actions,"  pp.  48-53. 


ENERGY  RELATIONS  65 

ments  of  catalytic  reactions  have  their  cause  either  in  a  lack  of 
understanding  of  the  definition  used,  or  an  attempt  to  modify  the 
definition  by  adding  thereto  in  place  of  redefining  the  original 
viewpoint.  In  any  event,  no  matter  what  definition  or  classifica- 
tion is  used  for  the  group  of  catalytic  reactions,  the  fact  remains 
that  the  general  principles  of  chemical  reactions  and  their  energy 
relations  must  hold  for  them  as  for  all  reactions.  Thus,  a  reac- 
tion which  proceeds  isothermally  and  at  constant  volume  will 
form  products  in  which  the  free  energy  of  the  reaction  mixture 
is  decreased.  This  decrease  in  free  energy  does  not  involve 
directly  the  question  of  rate  of  reaction  and  according  to  the 
older  viewpoints  would  not  be  included  in  a  discussion  of  cata- 
lytic actions.  At  the  same  time,  some  experimental  studies  have 
been  carried  out  which  are  of  interest  in  connection  with  the  pos- 
sibility of  linking  up  affinity  relationships,  reaction  velocities  and 
changes  in  these  by  suitable  catalysts,  and  structural  chemistry. 

The  work  of  Stieglitz  on  the  decomposition  of  imido  esters  and 
related  substances  and  the  increase  in  their  rates  of  decomposi- 
tion by  the  addition  of  acids  which  was  described  in  Chapter  II 
may  be  referred  to  again  in  this  connection.1  The  striking  ob- 
servation was  made  that  in  all  the  reactions  studied,  the  cata- 
lytic action  of  the  acid  was  found  to  be  intimately  connected 
with  the  transformation  in  acid  solution  of  the  positive  ion  of  a 
weaker  base  into  that  of  a  stronger  one  "the  results  no  doubt  of 
the  principle  of  the  loss  of  a  maximum  amount  of  free  energy." 
It  was  found  to  be  a  general  rule  in  these  changes  that  the  trans- 
formation "proceeds  with  the  greater  velocity  at  a  given  tem- 
perature the  weaker  the  original  base  is." 

A  few  of  the  reactions  studied  by  Stieglitz  may  be  quoted  in 
order  to  show  the  significance  of  these  relations  and  their  im- 
portance in  accounting  for  the  action  of  the  catalyst  in  some 
cases,  and  lack  of  action  in  others.  Imido  esters  are  rapidly  de- 
composed by  water  in  the  presence  of  acid,  because  the  ammon- 
ium hydroxide  which  is  the  base  corresponding  to  the  ammonium 
ion  formed  in  the  reaction  is  a  stronger  base  than  the  imido  ester 
base.  On  the  other  hand,  urea  ester  salts  are  quite  stable  in 
water  in  the  presence  of  acid.  The  urea  ester  base  is  a  stronger 
base  than  ammonium  hydroxide,  the  opposite  relation  to  the 

*J.  Stieglitz,  Jour.  Amer.  Chem.  Soc.  M,  221  (1910). 


66  CATALYTIC  ACTION 

first  example.  If  instead  of  a  urea  ester,  a  benzoyl  urea  ester, 
which  forms  a  weaker  base  than  ammonium  hydroxide,  is  used, 
the  decomposition  proceeds  smoothly  in  the  presence  of  acid. 
Similar  relations  were  found  to  hold  with  the  action  of  ammonia 
(or  ammonium  salts)  on  these  substances  in  place  of  water,  the 
positive  ion  of  the  weaker  base  being  transformed  into  the  posi- 
tive ion  of  a  stronger  base.  Thus,  imido  ester  salts  reacted 
with  ammonia  in  the  presence  of  acid  (or  with  ammonium  salts) 
to  form  amidines  (stronger  bases  than  imido  esters) ;  urea  ester 
salts  reacted  with  ammonia  to  form  guanidines,  in  contradistinc- 
tion to  their  lack  of  action  with  water,  since  urea  ester  bases  are 
stronger  bases  than  ammonium  hydroxide,  but  weaker  bases  than 
those  derived  from  guanidines,  etc. 

The  conclusions  of  Stieglitz  mark  a  definite  attempt  to  bring 
catalytic  actions  into  line  with  other  chemical  reactions  in  sug- 
gesting the  change  in  free  energy  as  a  classifying  principle.  Un- 
fortunately, data  are  not  as  yet  available  for  reactions  such  as 
have  been  described,  for  the  free  energy  to  be  calculated.  Un- 
questionably, in  the  course  of  time,  with  the  work  on  chemical 
equilibria  and  free  energy  or  chemical  affinity  measurements 
which  is  in  progress,  the  gaps  in  the  knowledge  of  such  relations 
will  be  filled  in  and  a  true  measure  of  the  affinity  changes  ob- 
tained. It  may  be  recalled  that  a  beginning  has  been  made  in 
such  studies  by  C.  G.  Derick1  in  following  rearrangements  of 
the  non-reversible  type,  of  compounds  which  spontaneously  go 
over  into  a  more  stable  state.  An  example  of  this  may  be  quoted 
in  the  rearrangement  of  A2>5-dihydroterephthalic  acid  to  the 
A1-5  isomer.  Heating  caused  this  reaction  to  go  to  completion  in 
a  short  time,  while  the  A1'5  acid  rearranged  to  the  A1-4  acid  when 
boiled  in  the  presence  of  hydroxyl  ions.  A  number  of  similar  ex- 
amples were  given.  These  rearrangements  took  place  slowly  in 
the  absence  of  a  catalyst  (heat  apparently  may  be  taken  to  pro- 
duce the  same  effect  as  a  catalyst) .  It  was  pointed  out  that  the 
criterion  of  stability  in  such  rearrangements  may  be  taken  to 
be  the  logarithm  of  the  ionization  constants  of  such  substances 
acting  as  acids  and  bases.  This  can  be  deduced  from  the  free 

1  Jour.  Amer.  Chem.  Soc.  S2,  1333  (1910). 


ENERGY  RELATIONS  67 

energy  expression  (of  the  second  law  of  thermodynamics) 

A  =  RTlogeK,  (5) 

in  which  K  is  the  ionization  constant  and  A  the  free  energy  of 
ionization,  or  the  change  in  free  energy  "as  expressed  by  the  re- 
action HA^±H+  +  A-  for  acids  and  ROH  *±  R+  +  OH-  for 
bases  when  the  initial  and  final  substances  are  at  unit  concen- 
tration. Therefore,  the  above  expression  gives  a  measure  of  the 
free  energy  of  ionization  in  terms  of  the  ionization  constant  and 
shows  that  the  free  energy  of  ionization  is  directly  proportional 
to  the  natural  logarithm  of  the  ionization  constant.  Since  that 
acid  is  most  stable  toward  ionization  which  possesses  the  small- 
est amount  of  free  energy  of  ionization,  it  is  evident  that  when 
one  acid  has  a  smaller  ionization  constant  than  another,  the 
change  in  free  energy  of  ionization  is  smaller  and  its  stability 
with  respect  to  ionization  is  greater."  The  result  of  the  com- 
parison of  the  ionization  constants  of  a  number  of  the  isomeric 
acids  showed  that  "the  free  energy  of  ionization  of  the  initial 
substance  is  greater  than  that  of  the  final  substance  and  that  dur- 
ing the  rearrangement  there  has  been  a  decrease  in  free  energy. 
.  .  .  The  final  (stable)  substance  in  a  series  of  true  rearrange- 
ments of  the  non-reversible  type  has  a  smaller  ionization  con- 
stant (or  free  energy  of  ionization)  and  the  logarithm  of  the 
ionization  constant  may  therefore  be  taken  as  a  criterion  of  sta- 
bility in  these  rearrangements." 

This  work  of  Derick  is  quoted  for  another  reason.  The  ion- 
ization constants  of  the  acids  decrease  as  they  rearrange  to  the 
more  stable  forms.  In  Stieglitz's  work  with  imido  esters  and 
analogous  compounds,  there  was  a  definite  tendency  for  the  re- 
actions to  take  place  in  the  sense  that  salts  of  stronger  or  more 
highly  ionized  bases  were  formed.  Stieglitz,  however,  was  not  in 
a  position  to  calculate  the  changes  in  free  energy  in  these  reac- 
tions. Both  sets  of  conclusions  are  based  upon  reliable  experi- 
mental work  and  cannot  therefore  be  questioned.  The  degrees 
of  ionization  of  the  salts  of  the  acids  and  bases  in  the  two  series 
of  experiments  do  not  enter  into  the  considerations.  The  obvious 
conclusion  is  reached  that  there  is  no  general  relation  with  regard 
to  ionization  constants  and  stability  in  determining  the  course  of 
a  reaction.  Each  reaction  studied,  and  the  exact  equilibrium  con- 


68  CATALYTIC  ACTION 

ditions  under  which  it  is  investigated,  must  be  considered  inde- 
pendently, and  conclusions  obtained  with  one  series  of  reactions 
cannot  be  carried  over  to  another  in  a  superficial  manner.  The 
one  fundamental  relation  will  hold,  that  in  every  case,  a  decrease 
in  free  energy  will  occur,  if  the  reaction  proceeds  isothermally 
and  at  constant  volume. 

The  relation  of  such  work  to  catalytic  reactions  will  become 
clearer  in  the  future  as  the  knowledge  increases  concerning  the 
mechanism  of  such  reactions,  their  equilibrium  conditions,  the 
free  energy  changes  derived  from  these,  and  the  factors  governing 
the  rate  of  attainment  of  equilibrium  and  the  actions  of  various 
substances  on  the  mechanism  and  rate. 

Before  leaving  the  work  of  Stieglitz  on  imido  esters,  refer- 
ence may  be  made  to  a  point  to  which  attention  was  already 
directed  in  previous  discussions.  In  the  decomposition  of  imido 
ester  with  water,  the  unionized  imido  ester  molecule  was  shown 
to  be  undergoing  change ;  in  acid  solution,  it  was  mainly  the  posi- 
tive imido  ester  complex  ion  which  reacted.  The  latter  reaction 
was  much  more  rapid  than  the  former,  and  the  increased  speed 
was  ascribed  to  the  catalytic  action  of  the  added  acid  (or  hydro- 
gen ion) .  As  stated  previously,  Stieglitz  pointed  out  that  in  acid 
solution,  the  unionized  molecule  reacted  as  well  as  the  positive 
ion,  that  the  concentration  of  the  latter  might  be  much  smaller 
than  that  of  the  former,  and  that  consequently  both  reactions 
could  be  observed  experimentally  simultaneously.  In  fact,  he 
showed  that  by  suitably  modifying  the  substances  he  could  vary 
the  extents  of  the  two  reactions  taking  place.  The  part  played 
by  the  acid  (or  hydrogen  ion)  in  the  reaction  was  perfectly 
definite  in  increasing  the  concentration  of  the  positive  imido 
ester  complex  ion. 

The  point  to  which  it  is  desired  to  call  attention  again  is 
whether  the  catalyzed  and  uncatalyzed  reactions  may  be  con- 
sidered to  be  the  same  in  the  sense  that  a  reaction  whose  velocity 
is  increased  by  a  catalyst  is  generally  considered  to  be  un- 
changed in  nature.  The  thorough  study  of  the  reaction  in  which 
an  imido  ester  is  decomposed  under  several  different  conditions 
has  shown  the  probable  mechanism  of  the  reaction  in  the  dif- 
ferent cases.  The  significance  of  the  term  catalyst  as  used  in 
the  older  (as  well  as  in  much  of  the  newer)  literature  seems  to 


ENERGY  RELATIONS  69 

be  indefinite.  The  acid  plays  a  definite  part  in  the  decomposi- 
tion of  the  imido  ester,  and  in  one  sense  it  is  present  at  the  end 
(combined  with  a  different  grouping).  The  mechanisms  of  the 
reactions  in  the  presence  and  absence  of  the  catalyst  are  quite 
different.  This  is  a  striking  example  of  the  apparent  lack  of 
need  for  the  conception  of  catalyst.  Careful  study  showed  ex- 
actly the  part  that  the  so-called  catalyst  played,  and  without 
using  the  conception  of  catalyst  and  treating  the  reaction  as  an 
ordinary  chemical  reaction,  ignoring  any  specific  virtue  or  harm 
which  might  be  ascribed  to  the  so-called  catalyst,  but  honestly 
admitting  ignorance  where  ignorance  exists,  it  appears  as  if  the 
knowledge  of  the  changes  involved  would  be  just  as  satisfactory 
as  when  more  or  less  imaginary  and  artificial  concepts  are 
invoked. 


Chapter  V. 
Recent  Theories  of  Chemical  Action 

The  development  in  recent  years  of  the  views  on  the  struc- 
ture of  matter  will  modify  profoundly  many  of  the  present  con- 
cepts of  science.  It  is  to  be  expected  that  some  of  these  theories, 
for  example  those  involving  the  structures  of  atoms  and  mole- 
cules, will  be  of  special  importance  to  chemistry,  and  will  result 
in  bringing  to  light  many  new  phenomena  and  will  account  for 
many  relations  heretofore  unexplained  or  explained  unsatis- 
factorily. It  is  true  that  these  newer  theories  have  not  as  yet 
taken  firm  foothold  in  chemistry.  The  attempts  to  apply  elec- 
tronic structures  to  molecules,  or  to  use  radiation  phenomena 
in  chemistry,  have  been  more  or  less  isolated,  but  there  can  be 
no  question  that  in  comparatively  few  years  these  fundamental 
relations  will  be  accepted  and  applied,  perhaps  in  somewhat 
modified  forms,  to  chemical  phenomena.  Chemical  reactions 
and  their  mechanisms  offer  an  inviting  field  for  the  application 
of  some  of  these  views.  Certain  theories  in  connection  with 
them  have  already  been  developed,  and  it  is  for  this  reason 
that  space  will  be  devoted  here  to  an  attempt  to  outline  some 
of  these  developments.  Catalytic  actions  as  a  group  of  chemi- 
cal reactions  would  naturally  be  considered  at  the  same  time. 
There  is  a  further  reason,  however,  for  including  catalytic  ac- 
tions in  these  discussions.  It  is  possible  that  the  views  on  radia- 
tion and  related  phenomena  which  are  coming  to  the  front  will 
furnish  a  new  and  satisfactory  explanation  for  many  of  the 
relations  heretofore  included  under  catalysis  and  in  this  way  do 
away  with  the  more  or  less  incomplete  definitions  and  classifica- 
tions which  have  been  proposed.  The  newer  views  and  concepts 
are  not  final  in  any  sense  nor  have  they  been  generally  adopted  or 
accepted  in  chemistry  even  where  applied.  The  fundamental 
theories  appear  to  be  sound,  and  it  is  quite  certain  that  the 
developments  from  these  will  be  of  value  ultimately,  even  if  the 

70 


RECENT  THEORIES  OF  CHEMICAL  ACTION       71 

suggestions  which  have  already  been  made  and  some  of  which 
will  be  presented  here,  should  prove  not  to  be  the  most  satis- 
factory. 

The  fact  that  the  velocity  of  a  monomolecular  reaction  is 
finite  can  only  signify,  and  this  has  been  pointed  out  repeatedly, 
that  the  molecules  of  the  substance  undergoing  change  are  not 
all  in  the  same  condition.  For  example,  in  speaking  of  the  rate 
of  decomposition  of  arsine,  van't  Hoff  stated:  x  ".  .  .  the  slow 
progress  of  such  a  monomolecular  reaction  shows  that  not  all 
the  molecules  of  a  gas  are  in  the  same  condition,  else  either  none 
would  be  decomposed  or  all  together."  In  a  reaction  of  higher 
order  where  two  or  more  molecules  react,  the  rate  of  reaction 
will  evidently  be  dependent  upon  the  frequency  of  the  collisions 
or  meetings  of  the  molecules  which  are  changed.  Such  meetings 
are  not  sufficient  in  themselves  for  reaction  to  occur.  In  addi- 
tion, the  molecules  must  be  in  the  necessary  state  or  condition 
as  with  a  monomolecular  reaction.  There  is  possibly  another 
factor  to  be  considered,  namely,  the  "steric"  factor,  according 
to  which  it  is  necessary  for  certain  portions  of  molecules  to  meet 
in  order  to  have  the  possibility  of  the  occurrence  of  a  reaction. 
This  will  be  considered  again  later. 

The  fact  that  the  velocity  of  a  reaction  of  the  second  or 
higher  order  is  not  alone  dependent  on  the  number  of  meetings 
of  the  molecules  can  be  shown  most  directly  by  the  effect  of 
increase  of  temperature.  The  kinetic  theory  of  gases  requires 
that  for  a  rise  of  10°  in  the  neighborhood  of  the  ordinary  tem- 
peratures the  number  of  collisions  per  unit  of  time  increase 
about  two  per  cent.2  Experimentally,  it  has  been  found  that 
the  velocities  of  many  chemical  reactions  are  increased  as  much 
as  two  to  three  fold  (200  to  300  per  cent)  by  10°  rise  in  tem- 
perature. It  is  evident,  therefore,  that  the  kinetic  theory  alone 
does  not  account  for  the  increase  in  reaction  velocity  with  tem- 
perature. Similarly,  other  physical  effects,  such  as  viscosity, 
are  insufficient  as  explanations.  There  must  be  involved  some 
more  deep-seated  change  within  the  molecules,  as  is  indicated  in 
monomolecular  reactions. 

1  "Lectures  on  Theoretical  and  Physical  Chemistry."     Translated  by  R.  A. 
Lehfeldt,   1898.     Vol.   1,   p.   192. 

2  Cf.  W.  C.  McC.  Lewis,  "A  System  of  Physical  Chemistry,"  Vol.  1,  "Kinetic 
Theory,"  1918,  pp.  409-10. 


72  CATALYTIC  ACTION 

Arrhenius *  considered  that  the  difference  in  the  condition 
of  the  molecules  could  be  accounted  for  by  assuming  two  types 
of  molecules  in  any  given  substance.  He  called  these  "active" 
and  "passive"  molecules  and  assumed  that  an  equilibrium  ex- 
isted between  them,  the  concentration  of  active  molecules  being 
small  as  compared  with  the  concentration  of  passive  molecules. 
Increase  in  temperature  would  increase  the  concentration  of 
active  molecules  and  in  this  way  account  for  the  great  increase 
in  reaction  velocity. 

This  view  of  active  and  passive  molecules,  suggested  by 
Arrhenius  in  1889  must  necessarily  be  true  since  it  is  funda- 
mentally a  statement  of  the  fact  that  all  of  the  molecules  of  a 
substance  do  not  react  simultaneously  in  a  monomolecular  reac- 
tion, and  that  the  number  of  collisions  in  a  bi-,  ter-,  etc.,  mole- 
cular reactions  are  not  the  only  factors  in  reactions  occurring 
with  these.  It  will  be  seen  .that  the  newer  theories  attempt 
explanations  of  the  facts  which  were  clearly  pointed  out  by 
Arrhenius  over  thirty  years  ago. 

Another  angle  from  which  to  consider  these  relations  was 
indicated  by  van't  Hoff:  2  "The  fact  that  a  reaction  needs  time 
for  its  completion  involves  that,  besides  the  cause  producing  it, 
which  we  may  describe  as  the  'moving  force'  or  'affinity/  a 
'resistance'  comes  into  play.  The  two  are  therefore  to  be  con- 
sidered separately,  and  it  may  be  foreseen  that  the  nature  of  the 
resistance  can  be  most  varied,  whilst  the  'moving  force'  is  definite 
for  a  given  state  of  matter."  This  has  been  expressed  at  various 
times  in  the  following  form  in  analogy  with  Ohm's  law  of  electric 
conduction: 

Tr  ,     .,        ,  T.       ,.  Driving  Force 

Velocity  of  Reaction  =  -  (1) 

Resistance 

The  driving  force  is  given  by  the  change  in  free  energy  of  the 
reaction.  The  meaning  of  the  resistance  term  is  not  in  itself 
very  clear.  Reasoning  in  a  circle,  with  the  driving  force  (or 
chemical  affinity)  constant,  the  change  in  velocity  is  a  measure 
of  the  resistance,  and  any  factors  which  change  the  chemical 
resistance  change  the  velocity.  The  effect  of  small  changes  in 

1  S.  A.  Arrhenius,  Z.  pliysik.  Chem.  4,  226   (1889). 
aJ.  H.  van't  Hoff,  I.e.  p.  176. 


RECENT  THEORIES  OF  CHEMICAL  ACTION      73 

temperature  may  be  said  to  be  sought  mainly  in  the  change  in 
resistance,  but  none  of  the  simple  effects  on  ordinary  or  com- 
mon types  of  resistance  can  be  used  to  explain  or  account  in 
any  satisfactory  manner  at  present  for  this  effect  on  the 
velocities  of  chemical  reactions. 

The  relations  which  have  been  presented  so  far  do  not  lead 
directly  to  a  chemical  interpretation  of  the  facts  but  at  the 
same  time  may  aid  in  fixing  the  nature  of  the  problem  to  be 
studied. 

The  study  of  the  atom  and  molecule  as  such  has  not  thrown 
light  on  this  problem.  However,  the  study  in  recent  years  of 
the  structure  of  the  atom  and  molecule  in  terms  of  negative 
electrons  and  positive  nuclei  and  their  possible  arrangements 
and  motions  has  made  it  probable  that  the  relations  developed 
in  this  way  will  ultimately  clear  up  a  number  of  these  ques- 
tions. A  comparison  of  rates  of  ordinary  chemical  reactions 
with  rates  of  decomposition  of  radioactive  substances  is  of  inter- 
est. The  former  are  affected  profoundly  by  rise  in  temperature, 
the  latter  not  at  all  as  far  as  experimental  observations  have 
gone.  The  latter,  however,  decompose  (or  react)  according 
to  the  monomolecular  reaction  velocity  law.  According  to  the 
modern  nuclear  atom  theory  the  difference  in  temperature  effect 
is  connected  with  the  fact  that  in  ordinary  chemical  reactions, 
the  negative  electrons  outside  the  nucleus  are  involved,  while 
in  radioactive  changes,  the  changes  which  take  place  occur  in 
the  positive  nucleus,  which  is  itself  made  up  of  various  positive 
charges  (hydrogen  and  helium  positively  charged  nuclei)  and 
negative  electrons.  In  considering  chemical  changes  here,  atten- 
tion will  be  devoted  solely  to  the  transformations  which  involve 
the  numbers,  positions  and  motions  of  the  negative  electrons 
outside  the  positive  nuclei  of  the  atoms  and  molecules.  The 
radioactive  changes  which  involve  numerical  changes  in  the 
charges  of  the  positive  nuclei  of  the  atoms  concerned  do  not 
come  within  the  scope  of  the  present  treatment,  although  the 
changes  are  known  in  a  number  of  cases  with  some  degree  of 
certainty. 

The  tendency  in  recent  years  to  ascribe  the  chemical  and 
physical  properties  of  atoms  and  molecules  to  electrons,  their 
arrangements,  and  motions,  is  involved  in -these  considerations. 


74  CATALYTIC  ACTION 

At  the  same  time,  it  must  be  stated  that  there  is  no  completely 
satisfactory  theory  at  hand  at  present.  Matters  are  in  a  state 
of  flux  and  development.  The  views  which  will  be  presented 
here  are  therefore  necessarily  fragmentary,  since  it  is  not  planned 
to  give  an  exhaustive  review  of  the  subject,  but  only  to  indicate 
some  of  the  possibilities  and  main  lines  of  development.  It  is 
hoped  that  enough  will  be  presented  to  awaken  interest  in  these 
problems  in  chemists  whose  work  may  lie  in  different  fields  and 
who  are  therefore  more  or  less  out  of  touch  with  such  views. 

The  negative  electrons  which  surround  the  positive  nucleus 
of  an  atom  (the  nucleus  may  itself  contain  negative  electrons 
and  it  is  therefore  necessary  to  indicate  which  are  meant)  are 
taken  to  be  the  active  agents  in  chemical  reactions.  J.  J.  Thom- 
son x  was  the  first  to  point  out  the  importance  of  electron  views 
in  the  molecular  structures  of  compounds  and  to  develop  certain 
relations  with  regard  to  the  spatial  arrangements  of  the  elec- 
trons in  atoms,  which  showed  interesting  analogies  with  a  num- 
ber of  chemical  and  physical  properties  of  series  of  elements. 
The  views  of  J.  J.  Thomson  with  regard  to  the  identity  of  the 
transfer  of  negative  electrons  between  atoms  with  valence  link- 
ings  were  applied  by  a  number  of  workers  to  the  structures  of 
chemical  compounds.  These  views  are  based  fundamentally 
upon  the  original  observations  of  Faraday  of  the  laws  of  electric 
conduction  in  solutions  and  the  conception  of  Helmholtz 2  in 
his  Faraday  lecture  of  1881  with  regard  to  the  atom  of  elec- 
tricity, but  made  definite  and  applicable  to  ordinary  chemical 
structures  by  the  experimental  and  theoretical  studies  of  J.  J. 
Thomson.  The  electron  valence  views  are  not  necessarily  con- 
nected with  any  particular  theory  of  the  possible  electron  con- 
figurations within  the  atom  or  molecule.  These  theories  and 
applications  of  the  electron  conception  of  valence  as  developed 
by  J.  M.  Nelson  and  the  writer,  beginning  in  1909,  purposely 
ignored  such  configurations.  There  was  a  great  temptation  to 
suggest  hypothetical  arrangements  of  electrons  which  could 
readily  have  been  brought  in  agreement  with  certain  properties 
of  atoms  and  molecules.  In  view  of  the  more  or  less  speculative 
nature,  at  the  time,  of  the  views  as  developed,  this  was  not  done. 

JJ.  J.  Thomson,  "The  Corpuscular  Theory  of  Matter,"  1904. 

2  H.  von  Helmholtz.     Vortriige  und  Rede,  J^te  Aufl.  2,  251  ;  Oes.  Abh.  S,  97. 


RECENT  THEORIES  OF  CHEMICAL  ACTION       75 

In  the  last  twelve  years  the  electron  conception  of  valence  has 
been  widely  accepted.  This  does  not  mean  that  the  theory  is 
in  any  way  final.  Much  remains  to  be  done  still  and  many  rela- 
tions to  be  unravelled,  but  it  appears  to  be  quite  certain  that  the 
electron  conception  of  valence  marked  a  definite  advance. 

As  the  usefulness  of  the  electron  conception  of  valence  be- 
came more  apparent,  and  the  nature  of  the  developments  became 
less  hypothetical,  the  next  step  was  taken  by  attempting  to 
develop  the  electronic  arrangements  within  the  atoms  and  mole- 
cules in  order  to  explain  and  account  for  chemical  and  also  physi- 
cal phenomena.  If  the  chemical  and  physical  properties  of  ele- 
ments and  compounds  can  be  shown  to  be  dependent  upon  certain 
definite  arrangements  of  electrons  in  atoms  and  molecules,  a 
great  advance  will  have  been  made.  Furthermore,  such  arrange- 
ments would  then  be  connected  with  chemical  changes  and  reac- 
tions and  it  would  be  possible  to  develop  theories  of  chemical 
reactions  upon  the  basis  of  electron  arrangements  and  their 
changes  within  the  molecules.  The  general  point  of  view  can 
be  stated  concisely.  The  molecule  is  taken  as  the  unit,  and  the 
arrangement  of  the  electrons  in  the  molecule  as  a  whole  con- 
sidered, at  least  in  gases  where  the  term  molecule  has  a  definite 
meaning.  No  distinction  is  made  as  to  the  electrons  which  were 
part  of  one  atom  or  another  before  the  combination  (and  accom- 
panying electronic  rearrangement)  to  form  the  molecule  took 
place.  The  stable  arrangement  of  the  electrons  in  the  molecule 
is  the  essential  feature  of  the  more  recent  developments.  They 
differ  from  the  electron  conception  of  valence  in  that  the  latter 
considered  the  change  which  occurs  in  an  atom  of  an  element 
before  and  after  combination,  emphasizing  this  change  by  indi- 
cating the  addition  or  subtraction  of  one  or  more  electrons 
(negative  or  positive  valence).  Conclusions  were  then  drawn 
with  regard  to  the  properties  of  the  resulting  combinations  from 
the  valence  (or  number  of  electrons  gained  or  lost)  and  the 
atoms  in  combination,  as  compared  with  the  uncombined  ele- 
ments. 

Both  methods  of  treatment  are  useful  and  should  aid  in  the 
development  of  theoretical  views.  They  are  not  directly  de- 
pendent upon  each  other,  although  the  molecular  electron  struc- 
tures may  be  considered  to  have  developed  as  a  result  of  the 


76  CATALYTIC  ACTION 

valence  electron  structures.  The  former  considers  the  molecule 
as  the  unit,  certain  general  arrangements  of  electrons  being 
capable  of  conferring  stability;  the  latter  considers  the  atoms 
plus  or  minus  valence  electrons  as  the  units. 

It  will  not  be  necessary  to  enter  farther  into  the  valence 
electron  structures  in  the  present  connection.  Reference  to  two 
recent  books  should  suffice  for  those  interested  in  this  part  of 
the  subject.1 

G.  N.  Lewis2  developed  the  idea  of  the  so-called  "cubical" 
atom  in  which  the  electrons  in  atoms  arrange  themselves  in  series 
of  concentric  shells,  these  electrons  being  stationary  within  limits. 
The  innermost  shell  contains  two  electrons,  all  other  shells  tend 
to  hold  eight,  each  at  a  corner  of  a  cube  or  in  pairs  at  the  corners 
of  a  regular  tetrahedron.  The  outermost  shell  may  hold,  2,  4,  6, 
or  8  electrons.  When  two  atoms  combine  to  form  a  molecule, 
the  tendency  in  the  molecule  is  to  form  the  cube  of  electrons. 
Two  or  more  electrons  may  be  common  to  two  cubical  arrange- 
ments. This  view  was  applied  to  a  number  of  physical  and 
chemical  problems  and  interesting  conclusions  obtained  espe- 
cially with  regard  to  certain  periodic  relations  which  occur  in 
various  series  of  elements. 

I.  Langmuir  extended  these  views  in  a  number  of  directions. 
It  will  be  impossible  in  the  space  available  here  to  give  even  a 
short  outline  of  the  applications  which  he  has  made  to  chemical 
and  physical  phenomena.  The  most  satisfactory  procedure 
seems  to  be  to  give  the  postulates  which  he  found  were  necessary 
in  order  to  develop  the  theory  of  the  molecular  electron  struc- 
tures. These  will  serve  to  show  the  underlying  concepts  and 
indicate  the  scope  of  the  views.  These  postulates  were  given  by 
him,  as  follows:  3 

"1.  The  electrons  in  atoms  are  either  stationary  or  rotate,  revolve,  or 
oscillate  about  definite  positions  in  the  atom.  In  the  most  stable  atoms, 
namely  those  of  the  inert  gases,  the  electrons  have  positions  symmetrical 
with  respect  to  a  plane,  called  the  equatorial  plane,  passing  through  the 
nucleus  at  the  center  of  the  atom.  No  electrons  lie  in  the  equatorial 
plane.  There  is  an  axis  of  symmetry  (polar  axis)  perpendicular  to  this 

1  H.  S.  Fry,  "The  Electronic  Conception  of  Valence  and  the  Constitution  of 
Benzene";    Longmans,    Green   &   Co.,    1921;    K.    G.   Falk,    "Chemical    Reactions; 
Their  Theory  and  Mechanism,"  1920. 

2  Jour.  Amer.   Chem.  Soc.  88,  762   (1916).     The  theory  of  W.  Kossel    (Ann 
Physik.  49,  229  (1916))  is  similar  to  that  of  Lewis  in  a  number  of  respects  and 
will  therefore  only  be  referred  to  in  this  connection. 

•I.  Langmuir,  Jour.  Amer.  CTiem.  /S'oc.  41,  868  (1919). 


RECENT  THEORIES  OF  CHEMICAL  ACTION       77 

plane  through  which  four  secondary  planes  of  symmetry  pass  forming 
angles  of  45°  with  each  other.  These  atoms  thus  have  the  symmetry  of 
a  tetragonal  crystal. 

"2.  The  electrons  in  any  given  atom  are  distributed  through  a  series  of 
concentric  (nearly)  spherical  shells,  all  of  equal  thickness.  Thus  the  mean 
radii  of  the  shells  form  an  arithmetic  series  1,  2,  3,  4,  and  the  effective  areas 
are  in  the  ratios  1;  22;  3a;  42. 

"3.  Each  shell  is  divided  into  cellular  spaces  or  cells  occupying  equal 
areas  in  their  respective  shells  and  distributed  over  the  surface  of  the  shells 
according  to  the  symmetry  required  by  Postulate  1.  The  first  shell  thus 
contains  2  cells,  the  second  8,  the  third  18,  and  the  fourth  32. 

"4.  Each  of  the  cells  in  the  first  shell  can  contain  only  one  electron,  but 
each  other  cell  can  contain  either  one  or  two.  All  the  inner  shells  must 
have  their  full  quotas  of  electrons  before  the  outside  shell  can  contain  any. 
No  cell  in  the  outside  layer  can  contain  two  electrons  until  all  the  other 
cells  in  this  layer  contain  at  least  one. 

"5.  Two  electrons  in  the  same  cell  do  not  repel  nor  attract  one  another 
with  strong  forces.  This  probably  means  that  there  is  a  magnetic  attraction 
(Parson's  magneton  theory)  which  nearly  counteracts  the  electrostatic 
repulsion. 

"6.  When  the  number  of  electrons  in  the  outside  layer  is  small  the 
arrangement  of  the  electrons  is  determined  by  the  (magnetic?)  attraction 
of  the  underlying  electrons.  But  when  the  number  of  electrons  increases, 
especially  when  the  layer  is  nearly  complete,  the  electrostatic  repulsion  of 
the  underlying  electrons  and  of  those  in  the  outside  shell  becomes  pre- 
dominant. 

"7.  The  properties  of  the  atoms  are  determined  primarily  by  the  num- 
ber and  arrangement  of  electrons  in  the  outside  shell  and  by  the  ease  with 
which  the  atom  is  able  to  revert  to  more  stable  forms  by  giving  up  or 
taking  up  electrons. 

"8.  The  stable  and  symmetrical  arrangements  of  electrons  correspond- 
ing to  the  inert  gases  are  characterized  by  strong  internal  and  weak  external 
fields  of  force.  The  smaller  the  atomic  number,  the  weaker  the  external 
field. 

"9.  The  most  stable  arrangement  of  electrons  is  that  of  the  pair  in  the 
helium  atom.  A  stable  pair  may  also  be  held  by  (a)  a  single  hydrogen 
nucleus;  (b)  two  hydrogen  nuclei;  (c)  a  hydrogen  nucleus  and  the  kernel 
of  another  atom;  (d)  two  atomic  kernels  (very  rare). 

"10.  The  next  most  stable  arrangement  of  electrons  is  the  octet,  that  is, 
a  group  of  eight  electrons  like  that  in  the  second  shell  of  the  neon  atom. 
Any  atom  with  atomic  number  less  than  20,  and  which  has  more  than  3 
electrons  in  its  outside  layer,  tends  to  take  up  enough  electrons  to  complete 
its  octet. 

"11.  Two  octets  may  hold  one,  two,  or  sometimes  three  pairs  of  electrons 
in  common.  One  octet  may  share  one,  two,  three,  or  four  pairs  of  its 
electrons  with  one,  two,  three,  or  four  other  octets.  One  or  more  pairs  of 
electrons  in  an  octet  may  be  shared  by  the  corresponding  number  of 
hydrogen  nuclei.  No  electron  can  be  shared  by  more  than  two  octets." 

These  postulates,  developed  as  the  result  of  the  study  of  the 
physical  and  chemical  properties  of  a  number  of  substances,  will 
serve  as  a  system  of  classification  of  the  properties  of  such  sub- 
stances. It  is  interesting  to  note  that  on  the  Lewis-Langmuir 
view,  a  single  valence  linking  is  represented  by  two  electrons 


78  CATALYTIC  ACTION 

held  in  common  by  the  two  atoms,  a  double  valence  by  four 
electrons,  etc.1 

Langmuir  also  indicated  the  contradiction  in  the  structure 
of  the  atom  on  the  octet  or  cubical  view  with  the  structure  of 
the  Bohr  atom  in  which  the  electrons  are  assumed  to  revolve 
in  one  plane  in  orbits  about  the  nucleus.  Since  the  Bohr  atom 
has  been  successful  in  connection  with  the  study  of  the  spectra 
of  hydrogen,  helium,  and  lithium,  possible  explanations  of  the 
contradiction  were  suggested. 

A  still  more  recent  theoretical  development  along  the  line  of 
atomic  and  molecular  electron  structure  is  that  of  J.  J.  Thom- 
son.2 The  electrons  in  an  atom  are  assumed  by  him  to  be  in 
equilibrium  because  of  their  mutual  repulsions  and  the  attraction 
of  the  positive  nucleus.  The  laws  governing  these  repulsions  and 
attractions  are  developed.  The  conditions  of  stability  in  the 
arrangement  of  electrons  around  a  central  positive  charge  are 
given.  It  is  shown  that  for  eight  electrons  the  cube  is  not  the 
stable  arrangement  but  rather  a  twisted  polyhedron  with  eight 
triangular  faces  and  two  four-sided  ones.  One  to  eight  electrons 
may  exist  on  the  surface  of  a  sphere  surrounding  a  correspond- 
ing positive  charge.  With  a  positive  charge  equal  to  nine,  eight 
electrons  will  form  a  spherical  shell  concentric  with  the  central 
charge,  and  one  electron  will  go  outside  to  find  a  position  of 
stable  equilibrium,  etc.  The  periodicity,  as  in  most  such  ar- 
rangements of  electrons,  is  similar  to  that  expressed  in  the  peri- 
odic system.  Other  regularities  are  brought  out.  Molecules 
are  formed  by  electrons  of  the  atoms  acting  as  couplings;  each 
valence  bond  involving  two  electrons,  one  from  each  atom.  The 
molecules  which  survive  (a  number  which  may  have  transitory 
existence  are  indicated)  are  those  which  show  the  smallest  ten- 
dency to  attract  other  molecules,  "the  law  of  survival  of  the 
unattractive."  A  number  of  structures,  both  simple  and  more 

1  For  various  developments  and  applications  of  these  views  cf.  among  others 
I.  Langmuir,  Jour.  Amer.  Chem.  Soc.  42,  274   (1920)  ;  W.  M.  Latimer  and  W.  H. 
Rodebush,  Jour.  Amer.   Ohem.   Soc.  J,2,  1419    (1920)  ;  R.   N.   Pease,  Jour.  Amer. 
Chem.  Soc.  43,  991  (1921)  ;  C.  R.  Bury,  Jour.  Amer.  Chem.  Soc.  43,  1602   (1921)  • 
E.   J.   Cuy,  Z.  Elektrochem.  27,  371    (1921)  ;   J.   R.   Partington,   Nature  107.   172 
(1921)  ;  A.  O.  Rankine,  Nature  107,  203   (1921)  ;  A.  W.  C.  Menzies,  Nature  107, 
331    (1921)  ;  H.   S.  King,  Jour.  Amer.   Chem.  Soc.  44,  323    (1922)  •  E    D    East- 
man, Jour.  Amer.  Chem.  Soc.  44,  438  (1922). 

2  "The   Structure   of  the   Molecule   and    Chemical   Combination,"   Phil    Mao 
(VI),  41,  510-544   (1921). 


RECENT  THEORIES  OF  CHEMICAL  ACTION       79 

complex  are  indicated,  and  other  properties  discussed,  but  it 
would  lead  too  far  to  enter  into  these  here. 

Very  recently,  G.  N.  Lewis  x  has  suggested  that  a  tetrahedral 
arrangement  of  the  electrons  in  the  outer  sphere  of  an  atom  with 
two  electrons  at  each  apex,  when  an  atom  is  in  combination, 
would  be  a  more  satisfactory  representation,  each  pair  of  elec- 
trons standing  for  a  chemical  linking. 

The  striking  features  of  these  various  developments  to 
which  it  is  desired  to  draw  attention  in  the  present  connection 
are,  first,  the  more  or  less  definite  structures  or  arrangements  of 
the  electrons  in  the  molecules  which  are  assumed  to  confer  sta- 
bility on  the  chemical  union  or  compound,  and  secondly,  taking 
into  account  the  relative  sizes  of  electrons,  nuclei,  atoms,  and 
molecules,  that  only  a  very  small  part  of  an  atom  is  actually 
directly  active  in  a  chemical  linking.  Chemical  architecture  has 
gone  beyond  the  atom  in  the  molecule  and  is  using  the  electron 
in  the  atom  and  in  the  molecule.  Whatever  directions  develop- 
ments in  these  lines  may  take,  electronic  structures  of  atoms 
and  molecules  will  surely  play  an  increasingly  important  part  in 
the  classification  of  chemical  compounds  and  reactions  and  in  help- 
ing to  answer  questions  involving  stability  and  reaction  velocity. 

These  views  of  electronic  structures  of  molecules  as  well  as 
of  atoms  represent  the  most  recent  developments  in  chemical 
structures.  A  possible  weakness  lies  in  the  fact  that  such  struc- 
tures apparently  assume  fixed  positions  for  the  outer  electrons. 
This  difficulty  was  of  course  clearly  recognized  by  Lewis  and  by 
Langmuir,  and  certain  assumptions  were  made  with  regard  to 
their  motions,  possibly  vibratory,  within  certain  limits  to  over- 
come this,  and  at  the  same  time  to  account  for  certain  physical 
properties  such  as  the  relative  positions  of  spectral  lines,  etc. 

It  is  interesting  to  note  that  the  number  eight  which  occurs 
so  frequently  in  these  electronic  structures,  first  in  the  J.  J. 
Thomson  model,  then  in  the  cubical  and  tetrahedral  models,  is 
the  modern  way  of  indicating  the  same  relation  which  led  D. 
Mendeleeff  as  a  result  of  the  grouping  of  the  elements  in  his 
Periodic  System  to  state  that  for  many  elements  which  combine 
with  oxygen  and  with  hydrogen,  the  sum  of  the  maximum  va- 

1  G.  N.  Lewis ;  Nichols  lecture,  delivered  before  the  New  York  Section  of  the 
American  Chemical  Society,  May,  1921. 


80  CATALYTIC  ACTION 

lences  which  any  one  element  shows  in  these  two  compounds  is 
eight.  The  same  fact  has  been  brought  out  repeatedly  in  dif- 
ferent ways  such  as  stating  that  the  extreme  difference  in  valence 
which  an  atom  can  show  is  eight  electrons,  etc.  The  spatial 
arrangements  are  in  the  first  instance  a  mechanical  model  of 
such  statements. 

Matter  is  considered  to  be  made  up  of  positive  units  and 
negative  units.  The  negative  unit  is  the  negative  electron  of  a 
certain  mass  and  size  with  a  unit  negative  charge.  The  positive 
unit  has  been  termed  the  "proton"  and  consists  of  a  hydrogen 
atom  with  a  unit  positive  charge,  a  mass  1800  times  as  large  as 
that  of  the  negative  electron  and  a  volume  1800  times  smaller. 
Striking  confirmation  of  the  general  theory  of  the  composition 
of  matter  was  given  recently  by  Rutherford.1  Rutherford  found 
experimentally  that  from  the  positive  nuclei  of  the  elements 
nitrogen,  boron,  fluorine,  sodium,  aluminium,  and  phosphorus,  by 
the  passage  of  swiftly  moving  a  particles  (from  radium  C  for 
example),  positively  charged  hydrogen  atoms  (protons)  were 
separated.  These  charged  hydrogen  atoms  moved  with  great 
speeds,  showing  penetrating  powers  of  from  40  centimeters  to 
90  centimeters  in  air.  Such  hydrogen  atoms  carrying  unit  posi- 
tive charges  could  only  have  come  from  the  disintegration  of 
the  positive  nuclei  of  the  indicated  elements.  Of  the  elements 
examined  in  this  way,  only  the  six  mentioned  above  could  be 
broken  up.  The  atomic  masses  of  these  elements  are  given  by 
the  expressions  4n  +  2  or  4n  +  3,  in  which  n  is  a  whole  number. 
Elements  of  mass  4n  (such  as  carbon,  oxygen,  and  sulfur)  showed 
no  effect.  The  explanation  was  advanced  that  the  nuclei  of  the 
six  elements  decomposed  are  built  up  of  helium  nuclei  (mass  4) 
and  of  hydrogen  nuclei. 

The  present  view  of  the  atom,  then,  pictures  a  complex  system 
consisting  of  a  central  region  in  which  is  concentrated  a  certain 
number  of  positive  charges  and  negative  electrons,  the  number 
of  positive  charges  being  in  excess  of  the  negative,  and  an  outer 
region  containing  a  number  of  negative  electrons  corresponding 
to  the  excess  positive  charge  of  the  central  region,  arranged  in 
certain  configurations  and  with  more  or  less  restricted  motions. 

*E.  Rutherford,  Phil.  Mag.  87,  538  (1919);  Bakerian  Lecture,  Proc  Roy 
Soc.  London  (A)  97,  374  (1920)  ;  E.  Rutherford  and  J.  Chadwick,  Phil.  Mag.  &t 


RECENT  THEORIES  OF  CHEMICAL  ACTION       81 

The  actual  volume  of  the  atom  is  that  included  within  the  sphere 
of  action,  if  it  may  be  called  so,  of  the  system  of  positive  nu- 
cleus plus  surrounding  electrons.  Because  of  the  small  volumes 
of  the  nucleus  and  electrons,  the  actual  volume  of  the  atom  so 
occupied  will  be  very  minute  with  comparatively  immense  empty 
spaces  composing  the  major  part  of  the  atom.  The  forces, 
whether  gravitational,  electrical,  or  of  other  nature,  which  act  to 
maintain  the  compositions  and  configurations,  do  not  enter 
directly  at  present  into  the  problem  under  discussion.  The  point 
to  which  it  is  desired  to  call  attention  is  that  in  such  a  structure, 
which  may  be  of  varying  and  considerable  complexity  with  dif- 
ferent atoms,  it  is  readily  conceivable  that  a  certain  part  of  the 
atom  might  react  and  not  the  rest.  This  is  comparable  to  those 
reactions  of  organic  and  also  other  compounds,  in  which,  by  a 
suitable  choice  of  reagents  and  conditions  it  is  possible  to  bring 
about  a  reaction  with  an  atom  or  grouping  of  that  compound 
and  leave  the  rest  of  the  molecule  apparently  unaffected.  Views 
similar  to  this  have  been  considered  by  Trautz  1  and  by  Stern 2 
as  necessary  in  the  reactions  between  two  or  more  molecules, 
where,  not  only  must  there  be  a  meeting  of  the  two  in  order 
to  have  reaction  possible,  but  the  meeting  must  take  place  be- 
tween two  definite  parts  of  the  molecule.  This  factor,  called  the 
"steric"  factor  is  not  considered  to  be  the  same  as  saying  that 
the  molecules  must  be  in  an  active  state,  but  is  an  additional 
factor.  Further,  it  may  also  be  pointed  out  that  atoms  in  com- 
plex molecules  are  not  assumed  to  be  fixed  in  position,  but  have 
a  certain  motion  of  their  own.  The  only  requirement  in  the 
chemical  molecule  is  that  the  atoms  are  not  free  to  take  up  any 
motion  whatsoever,  but  are  controlled  by  certain  other  atoms 
with  which  they  are  said  to  be  directly  combined.  This  com- 
bination involves  a  rearrangement  of  negative  electrons  outside 
the  nucleus,  either  in  the  general  terms  in  which  the  exact  posi- 
tions are  not  specified  as  in  the  electron  valence  theory,  or  in 
the  more  specific  configurations  postulated  in  the  different  mole- 
cular electron  theories.  The  motions  of  these  atoms  as  such 
might  be  considerable.  The  same  is  true  for  the  arrangements 
and  motions  of  the  electrons  in  an  atom. 

*M.  Trautz,  Z.  anorg.  Chem.  106,  149  (1919). 
8O.  Stern,  Awn.  Phys.  44,  497  (1914). 


82  CATALYTIC  ACTION 

The  electrons  in  a  molecule  are  considered  to  be  arranged 
in  definite  more  or  less  stable  configurations  which  determine 
the  chemical  and  physical  properties  of  the  substances.  Here 
again,  reaction  may  occur  because  of  certain  electrons,  or  elec- 
tron arrangements,  or  electron  motions.  This  leads  to  a  possible 
suggestion  with  regard  to  the  rate  of  reaction  of  a  single  sub- 
stance undergoing  change.  This  substance  possesses  certain  elec- 
tron arrangements  and  these  electrons  have  motions  of  their 
own,  possibly  vibratory,  within  limits.  As  long  as  a  certain 
configuration  persists,  the  substance  is  stable,  but  if  in  the  inter- 
nal motions  of  the  electrons  more  stable  configurations  are 
reached  in  certain  instances,  in  which  possibly  a  breaking  up  of 
certain  atomic  linkings  or  rearrangement  of  valence  electrons 
occurs,  then  so-called  chemical  reaction  is  said  to  take  place.  In 
such  an  explanation  of  chemical  action,  the  question  of  velocity 
of  motion  of  the  electrons  does  not  appear  to  be  involved  neces- 
sarily, although  it  is  probable  that  it  may  play  a  part.  It  is 
essentially  a  question  of  configurations  of  varying  degrees  of 
stability.  More  complex  molecules  should  show  more  possibil- 
ities of  reaching  different  configurations  of  greater  stability  than 
less  complex  molecules  and  might  therefore  show  greater  pos- 
sibilities of  reaction.  In  the  absence  of  external  influences,  with 
a  given  species  of  molecules,  it  is  a  question  of  chance,  or  better 
perhaps,  of  probability,  as  to  the  number  of  molecules  which 
would  be  in  the  favored  configuration  for  reaction  at  any  instant. 
This  hypothetical  view  may  be  the  explanation  for  the  mono- 
molecular  reaction  velocity  law.  The  views  involving  radiation 
phenomena  which  will  be  presented  presently,  appear  to  be  based 
upon  such  considerations  if  a  mechanical  model  of  the  atom  or 
molecule  is  thought  of,  but  in  the  actual  developments  such 
models  are  ignored,  and  the  theoretical  concepts  are  independent 
of  these  configurations,  at  the  present  time,  at  any  rate. 

Any  factor,  therefore,  which  may  influence  the  electron  con- 
figurations and  limited  motions,  may  influence  the  rate  of  change 
into  more  stable  configurations.  The  predominating  influence  is 
here  placed  on  the  outside  negative  electrons.  The  positive 
nuclei  are  not  considered  as  taking  a  prominent  part  in  the 
chemical  reactions.  They  are  assumed  simply  to  be  associated 
with  the  negative  electrons.  At  the  same  time  "it  must  be  stated 


RECENT  THEORIES  OF  CHEMICAL  ACTION       83 

that  while  attention  is  just  now  fixed  on  the  negative  electrons, 
their  configurations  and  motions,  as  the  dominating  feature  in 
chemical  reactions,  it  must  be  remembered  that  these  configura- 
tions and  motions  are  controlled  by  the  positive  nuclei.  Some 
time  in  the  future  the  point  of  view  may  be  shifted  and  the  prop- 
erties of  the  nuclei  used  as  the  underlying  causes  for  the  various 
phenomena. 

In  studying  the  arrangements  and  motions  of  electrons  in 
atoms  and  molecules,  the  properties  which  appear  to  be  of  most 
value  are  those  involving  the  various  forms  of  radiation.  Radia- 
tion phenomena  appear  to  offer  the  most  hope  for  a  successful 
study  of  the  properties  of  the  electrons  within  atoms  and  mole- 
cules, including  under  these  properties  such  factors  as  the  mo- 
tions and  arrangements  of  the  electrons  and  the  changes  in  these 
motions  and  arrangements  under  the  influence  of  outside  agen- 
cies. Considerable  work  is  being  carried  on  at  the  present  time 
on  radiation  problems  in  connection  with  chemical  changes. 
Some  of  the  theoretical  developments  which  have  resulted  from 
such  studies  will  be  presented.  It  will  be  impossible  to  give  a 
definite  statement  of  the  present  status  of  the  subject  with  regard 
to  final  or  even  satisfactory  theories.  Work  is  in  progress  in  a 
number  of  places,  and  the  views  presented  may,  in  part,  be 
contradictory,  but  they  will  serve  to  show  the  trend  the  work  is 
taking,  and  also  possibly  indicate  solutions  to  certain  definite 
problems. 

Arrhenius  *  showed  in  1889  that  the  effect  of  temperature  on 
the  reaction  velocity  could  be  represented  by  the  equation 

^?2*  =  Ji  •       (2) 

dT  T2 

or,  in  the  integrated  form, 

0) 


in  which  k  denotes  the  reaction  velocity  constant  at  the  absolute 
temperature  T,  or  k^  and  /c2  the  velocity  constants  at  tempera- 

1  S.  A.  Arrhenius,  Z.  physik.  Ofiem.   >,,  226   (1889).     Cf.  also  J3.  C.  C,  Baly 
ana  F.  O,  Rice,  J.  Chem.  -S'?c.  101,  1475  (1912), 


84  CATALYTIC  ACTION 

tures  T!  and  T2,  and  A,  a  constant  which  is  very  nearly  inde- 
pendent of  the  temperature  for  a  given  reaction. 

Equations  (2)  and  (3)  are  similar  in  form  to  the  van't  Hoff 
equation  connecting  the  heat  evolved  in  a  chemical  reaction 
with  the  equilibrium  constant  and  the  temperature.  Arrhenius 
deduced  his  equation  on  the  basis  of  the  presence  of  passive  and 
active  molecules  and  of  the  existence  of  an  equilibrium  between 
the  two  forms,  the  term  A  representing  one-half  the  energy 
required  to  transform  one  mol  of  the  passive  form  of  the  reacting 
molecules  into  the  active  form  (or  AR  representing  the  energy  to 
transform  one  mol  in  this  way).  The  Arrhenius  equation  holds 
fairly  satisfactorily  for  a  number  of  reactions  which  have  been 
studied,  but  the  theoretical  assumptions  which  appear  to  be 
involved  in  the  deductions  are  somewhat  uncertain.  The  equa- 
tion, while  representing  the  experimental  facts,  must  therefore 
be  looked  upon  as  empirical  in  so  far  as  the  deductions  of 
Arrhenius  are  concerned. 

Much  of  the  more  recent  work  on  these  questions  has  in- 
volved expressions  of  the  form  of  the  Arrhenius  equation,  which 
appears  to  represent  the  facts  in  a  satisfactory  manner,  together 
with  attempts  to  base  these  upon  more  satisfactory  bases  or 
assumptions.  From  the  point  of  view  of  statistical  mechanics, 
Marcelin  *  and  Rice  2  showed  that  the  relation  between  reaction 
velocity  and  temperature  may  be  given  by  the  equation 

d  log,  k        E 


_. 
dT        ~RT2 

In  their  deductions  it  was  assumed  that  the  condition  for  a 
molecule  reacting  is  brought  about  by  the  attainment  of  a  certain 
critical  value  for  its  internal  energy.  The  term  E  in  the  equation 
denotes  the  difference  between  this  critical  value  and  the  average 
internal  energy  per  gram  molecule,  and  was  therefore  later 
called  by  W.  C.  McC.  Lewis  the  "critical  increment."  As 
developed  by  Rice,  E  in  equation  (4)  is  given  by  the  expression 
Vc  —  Vm  +  1/2  RT,  in  which  Vc  equals  the  critical  value  of  the 
internal  energy  of  the  molecules,  and  Vm  the  mean  value  of  the 
potential  energy  of  the  molecules.  These  deductions  did  not  sug- 

1  R.  Marcelin,  Ann.  phys.  S,  120  (1915). 

2  F.  O,  Rice,  Rep.  Brit.  Ass'n.  1915,  317, 


RECENT  THEORIES  OF  CHEMICAL  ACTION       85 

gest  any  mechanism  by  which  this  increase  in  energy  from  the 
mean  to  the  critical  value  is  brought  about.  Such  a  possible 
mechanism  was  given  in  a  series  of  papers  by  W.  C.  McC.  Lewis 
and  tested  experimentally  in  a  number  of  ways.  "The  hypothesis 
is  that  the  increase  in  internal  energy  which  a  molecule  must 
receive  before  it  is  capable  of  reacting  (that  is,  the  critical  incre- 
ment E)  is  communicated  to  it  by  infra-red  radiant  energy  pres- 
ent in  the  system,  the  addition  of  energy  being  made  in  terms  of 
quanta  of  the  absorbable  type.  .  .  .  The  principal  feature  of  the 
hypothesis  is  the  introduction  of  the  quantum  theory  into  the 
problem  of  chemical  kinetics,  the  effect  of  temperature,  and 
catalysis."  1  Lewis  adopted  the  view  of  the  action  of  infra-red 
radiation  "which  was  necessarily  present  throughout  any  system 
in  virtue  of  its  temperature"  as  possibly  "the  active  agent  in  pro- 
moting chemical  changes  of  the  ordinary  or  thermal  type,  such 
changes  including  not  only  the  so-called  uncatalysed  reactions, 
but  catalysed  reactions  as  well,  in  so  far  at  least  as  these  are 
met  with  in  homogeneous  systems,  and  further  that  the  influence 
of  temperature  on  velocity  might  be  explained  on  the  same  basis." 
Lewis  also  showed  that 

E  =  N  h  Y  (5) 

in  which  N  equals  the  number  of  molecules  in  one  gram  molecule 
of  substance  (6.062  X  1023) ,  h,  Planck's  constant  (6.5  X  lO"27 
erg- seconds ),  and  y  a  single  frequency  of  vibration  which  was 
taken  to  be  the  effective  frequency  in  the  change  which  was  oc- 
curring. This  expression  is  a  statement  of  the  Einstein  law  of 
the  photochemical  equivalence  applied  to  infra-red  radiation. 

According  to  the  view  of  Lewis,  in  catalytic  actions,  the 
catalyst  absorbs  the  infra-red  radiation  and  transmits  this  energy 
to  the  reacting  molecules  which  are  themselves  not  capable  of 
absorbing  these  radiations.  This  brings  catalytic  actions  in  line 
with  chemical  reactions  in  general,  except  that  with  reactions 
not  included  under  catalysis  the  reacting  molecules  themselves 
absorb  the  infra-red  radiation  directly. 

The  experimental  verification  of  Lewis'  original  views  was 
not  entirely  satisfactory.  For  a  bimolecular  homogeneous  reac- 
tion, such  as  the  decomposition  of  'gaseous  hydrogen  iodide, 

*W.  C.  McC.  Lewis,  J.  Chem.  800.  109,  798  (1916). 


86  CATALYTIC  ACTION 

fairly  good  agreement  was  obtained  between  the  energy  incre- 
ment E  as  calculated  by  means  of  the  reaction  velocity  constants 
for  two  different  temperatures,  and  as  calculated  from  the  infra- 
red vibration  frequency  on  the  basis  of  the  Einstein  law.  On 
the  other  hand,  the  agreement  for  the  monomolecular  reactions 
was  not  nearly  as  satisfactory,  and  in  fact,  in  some  cases  such 
as  the  rate  of  dissociation  of  phosphine,  the  disagreement  was 
too  great  (of  the  order  of  107)  for  the  relation  to  be  considered 
valid.  It  must  be  mentioned,  however,  that  in  these  deductions 
the  index  of  refraction  of  the  mixture  was  assumed  to  be  unity. 
This  factor  was  brought  out  in  more  recent  publications  by 
Lewis  as  will  be  mentioned  presently. 

Equation  (4)  written  in  different  forms  has  formed  the  basis 
for  investigations  by  a  number  of  workers  in  the  last  years. 
The  formulation  employed  most  frequently  involved  the  term  Q 
in  place  of  E  and  the  expression  of  the  equation  in  the  exponen- 
tial form  is  as  follows: 

k  =  se-Q/RT  (6) 

in  which  k,  as  before,  denotes  the  specific  reaction  rate,  s  a  quan- 
tity with  the  dimensions  of  time,  and  Q,  a  quantity  with  the 
dimensions  of  energy  and  found  experimentally  to  be  very  nearly 
constant  within  certain  more  or  less  extended  ranges  of  temper- 
ature, and  known  variously  as  "heat  of  activation,"  "energy  of 
activation,"  and  "critical  increment." 

The  quantity  Q  has  been  interpreted  in  various  ways.  For 
example,  Arrhenius,  as  already  pointed  out,  considered  it  to  be 
the  heat  of  reaction  accompanying  the  transformation  of  the 
"passive"  form  of  molecules  into  the  "active"  form;  Marcelin 
and  Rice,  the  energy  of  their  "active"  state  minus  their  mean 
energy;  Trautz,1  one  of  the  first  workers  to  consider  the  infra- 
red radiation  as  playing  an  important  part  in  chemical  actions, 
calculated  this  "heat  of  activation"  from  the  "heats  of  activa- 
tion" of  individual  atoms  by  empirical  rules;  W.  C.  McC.  Lewis 
and  Perrin  2  placed  Q  =  Nhy  =  the  radiant  energy  of  activating 
frequency  y  which  must  be  absorbed  to  put  the  molecules  into 

1Cf.  Z.  anorg.  Chem.  102,  81  (1918),  for  a  summary  of  the  work  of  Trautz, 
also  Ibid.,  106,  149  (1919). 

2J.  Perrin,  Ann.  phys.   [9]  11,  5   (1919). 


RECENT  THEORIES  OF  CHEMICAL  ACTION       87 

the  reactive  condition  ;  Tolman  l  on  a  statistical  mechanical 
basis  puts  Q  equal  to  the  mean  energy  of  molecules  and  modes 
of  electromagnetic  vibration  when  these  take  part  in  a  reaction 
minus  their  mean  energy  whether  or  not  they  are  in  a  reactive 
condition;  and  S.  Dushman  2  placed  Q—  Nhy  similarly  but  speci- 
fied no  mechanism  of  activation. 

With  regard  to  the  significance  of  the  term  s  which  has  the 
dimensions  of  frequency,  for  a  monomolecular  reaction  Trautz 
developed  an  equation  in  which  its  value  depended  on  the  num- 
ber of  collisions  between  the  constituent  parts  within  the  mole- 
cule. Lewis  developed  several  different  ways  of  evaluating  s. 
The  recent  suggestion  of  Dushman,  placing  s  equal  to  y  sug- 
gested interesting  possibilities.  Tolman  3  pointed  out  in  this 
connection  the  probable  limitations  of  Bushman's  method  of 
procedure,  and  that  Q  is  in  general  not  constant  nor  does  it  cor- 
respond to  a  single  vibration  frequency.  He  showed  that  using 
the  assumptions  given  in  discussing  the  value  of  Q, 


(7) 


and  assuming  Q  to  be  constant,  that 

k  =  se-Q/RT          '.,  .        (6) 

in  which  s  is  also  constant.     In  general,  however, 

k=e  e-Q/RT=se-Q/RT  (8) 


in  which  s  =  e'U^is  a  variable  and  therefore  could  not  be  put 
J  Kl 

equal  to  a  single  frequency,  as  Dushman  assumed.  For  bimole- 
cular  reactions,  s  has  been  put  proportional  to  the  number  of 
collisions  between  the  molecules  of  the  substances  reacting  at 
the  temperature  in  question.  This  proportionality,  however,  in- 
cludes the  factor  of  the  state  of  the  molecules  spoken  of  in  con- 
nection with  monomolecular  reactions. 

1  R.  C.  Tolman,  Jour.  Amer.  Chem.  Soc.  42,  2506  (1920). 

2  S.  Dushman,  Jour.  Amer.  Chem.  Soc.  43,  397   (1921). 
SR.  C.  Tolman,  Jour.  Amer.  Chem.  Soc.  43,  269   (1921). 


88  CATALYTIC  ACTION 

The  experimental  material  which  is  available  to  test  the 
equations  which  have  been  given  is  rather  scant.  The  general 
equation  which  is  used  has  the  following  form 

(9) 

as  already  indicated,  in  which,  from  quantum  theory  considera- 
tions, Q  =  Nhy  =  energy  of  activation  of  one  gram  molecule  of 
substance.  To  test  this  equation,  values  of  k  determined  experi- 
mentally at  different  temperatures  may  be  used,  and  the  value 
of  Q  determined.  From  this,  the  radiation  frequency  y  which 
should  be  the  activating  radiation  may  be  calculated.  On  the 
other  hand,  from  the  absorption  spectrum  of  the  substance,  it 
should  be  possible  to  calculate  the  velocity  constant  of  the 
reaction. 

A  few  reactions  occurring  in  the  gaseous  phase  have  been 
studied  in  this  way,  but  the  data  are  not  complete  enough  for 
a  general  discussion.1  In  fact,  it  seems  to  be  still  necessary  to 
make  certain  assumptions  in  applying  the  theoretical  equations 
to  the  experimental  results.  Thus  Tolman 2  showed,  in  reviewing 
the  previous  work,  that  the  views  of  Perrin  (and  also  of  others) 
involved  the  assumption  that  in  activating  a  substance,  light  of 
a  single  frequency  or  of  a  very  narrow  range  of  frequencies  only 
would  be  involved.  Experimentally  it  has  been  found  that 
photochemical  reactions  are  brought  about  by  light  over  a  con- 
siderable range  above  the  limiting  threshold  frequency.  Lang- 
muir3  showed  that  the  two  tests  of  the  radiation  hypothesis, 
(1)  that  the  reacting  substance  must  absorb  radiation  of  the 
frequency  required  to  produce  activation  and  that  therefore 
there  must  be  an  absorption  band  which  includes  this  fre- 
quency, and  (2),  that  the  total  amount  of  radiant  energy  ab- 
sorbed must  be  sufficient  to  supply  the  known  heat  (or  energy) 
of  activation  to  the  molecules  which  reacted,  do  not  hold  with 
the  reactions  for  which  data  were  available  (decomposition  of 
phosphine,  dissociation  of  hydrogen,  iodine,  and  of  nitrogen 
peroxide) .  He  considered  that  the  form  of  the  Arrhenius  equa- 

1  Cf.  W.  C.  McC.  Lewis,   Trautz,  Tolman,  etc.,   also  F.   Daniels   and   E.   M. 
Johnston,  Jour.  Amer.  Chem.  Soc.  43,  53,  72    (1921),  for  results  on  the  decom- 
position of  nitrogen  pentoxide. 

2  R.  C.  Tolman,  Jour.  Amer.  Cliem.  Soc.  42,  2506  (1920). 
•I.  Langmuir,  Jour.  Amer.  Chem.  Soc.  J#t  2190  (1920). 


RECENT  THEORIES  OF  CHEMICAL  ACTION       89 

tion  (equation  (1))  was  due  to  its  being  based  upon  statistical 
laws,  assuming  "that  the  chance  that  a  given  molecule  will 
undergo  a  chemical  reaction  is  proportional  to  the  probability 
that  it  contains  a  specific  amount  of  energy."  "The  similarity 
between  the  Arrhenius  and  the  Wien  (radiation)  equation  thus 
results  from  the  fact  that  reaction  velocity  and  radiation  are 
fundamentally  dependent  upon  phenomena  involving  probability. 
Both  equations  can  be  derived  from  the  same  statistical  law  .  .  . 
the  energy  for  activation  of  molecules  must  be  derived  from 
internal  energy  of  the  molecules."  Langmuir  also  spoke  of  a 
sort  of  "trigger"  action  which  is  necessary  to  bring  about  chemi- 
cal action  in  a  molecule.  Tolman,  on  the  other  hand,  attempted 
to  overcome  the  difficulties  by  a  more  general  derivation  of  the 
kinetic  equations  on  the  basis  of  statistical  mechanics,  but  did 
not  apply  his  deductions  to  definite  cases. 

Baly  x  has  taken  hold  of  the  problem  from  a  different  angle. 
His  fundamental  premise  is  that  "the  observed  deviations  from 
Einstein's  law  of  photochemical  equivalence  might  very  possibly 
be  due  to  the  re-absorption  by  the  surrounding  reactant  mole- 
cules of  the  energy  radiated  during  the  reaction."  "According 
to  this  law,  when  a  reaction  takes  place  photochemically,  the 
absorption  by  a  molecule  of  an  amount  of  energy  equal  to  one 
quantum  at  its  absorbing  frequency  results  in  that  molecule 
undergoing  reaction,  and  the  number  of  quanta  absorbed  and 
the  number  of  molecules  reacting  must  be  equal.  A  study  of 
many  photochemical  reactions  has,  however,  shown  that  except 
in  one  case  the  number  of  molecules  reacting  is  far  in  excess  of 
the  number  of  quanta  absorbed."  "In  any  photochemical  reac- 
tion it  is  obvious  that  the  first  stage  is  the  absorption  of  the 
light  energy,  and  that  the  whole  can  be  written 

A+E=B+E+K 

where  A  and  B  are  the  reactant  and  resultant  molecules  re- 
spectively, E  is  the  amount  of  light  energy  absorbed,  and  K  is 
the  ordinary  observed  heat  of  the  reaction.  Now  the  molecule  A 

1E.  C.  C.  Baly,  Phil.  Mag.  (VI)  40,  15  (1920)  ;  E.  C.  C.  Baly  and  W.  F. 
Barker,  J.  Chem.  Soc.  119,  653  (1921)  ;  cf.  also  F.  Daniels  and  E.  M.  Johnston 
Jour.  Amer.  Chem.  Soc.  $,  53,  72  (1921)  on  the  thermal  and  photo-chemical 
decomposition  of  nitrogen  pentoxide,  and  E.  C.  C.  Baly,  I.  M.  Heilbron  and 
W.  F.  Barker,  J.  Chem.  Soc.  119,  1025  (1921),  on  "The  Synthesis  of  Formalde- 
hyde and  Carbohydrates  from  Carbon  Dioxide  and  Water." 


90  CATALYTIC  ACTION 

absorbs  the  energy  E  at  its  characteristic  frequency,  and  the 
minimum  value  of  E  is  one  quantum  at  that  frequency.  On 
the  other  hand,  it  is  evident  that  the  whole  of  the  energy  on  the 
right  hand  side  must  be  radiated  at  frequencies  characteristic  of 
the  resultant  molecule  B,  that  is  to  say,  the  total  energy  radi- 
ated during  the  reaction  must  be  equal  to  an  integral  number 
of  quanta  at  frequencies  characteristic  of  B.  It  has  previously 
been  shown  that  the  frequencies  of  any  molecule  are  integral 
multiples  of  the  frequencies  of  its  atoms,  and  since  the  molecules 
A  and  B  have  at  any  rate  some  atoms  in  common,  it  follows 
that  they  must  have  frequencies  in  common.  It  was  therefore 
suggested  that  whereas  Einstein's  law  must  hold  if  the  energy 
radiated  during  a  reaction  is  dissipated  to  the  surroundings,  the 
fact  that  A  and  B  have  frequencies  in  common  will  result  in 
some  of  this  energy  being  absorbed  by  further  molecules  of  A. 
If  the  amount  thus  reabsorbed  is  sufficiently  great,  more  than 
one  molecule  of  A  will  react  for  every  quantum  absorbed." 
This  view  was  tested  by  means  of  the  photochemical  reaction 
between  hydrogen  and  chlorine,  for  which  Einstein's  law  had 
been  found  not  to  hold.  It  was  found  that  with  a  given  light 
intensity  the  amount  of  hydrogen  chloride  formed  in  unit  time 
was  at  first  small  and  then  rapidly  increased  up  to  a  constant 
maximum.  The  constant  maximum  rate  of  formation  of  hydro- 
gen chloride  was  not  proportional  to  the  intensity  of  the  light. 
The  divergence  from  Einstein's  law  of  photochemical  equiva- 
lence depended  on  the  intensity  of  the  light  and  rapidly  increased 
with  the  intensity. 

Baly  concluded  that  the  evidence  for  the  reabsorption  of 
radiated  energy  in  a  chemical  reaction  was  satisfactory  and 
accounted  for  the  fact  that  the  laws  developed  from  the  quantum 
theory  and  the  radiation  hypothesis  did  not  hold.  This  view 
leads  to  new  and  striking  developments  with  regard  to  these 
relations,  and  while  apparently  not  simplifying  them,  promises 
to  furnish  more  satisfactory  bases  for  the  theoretical  and  experi- 
mental developments. 

A  somewhat  different  explanation  of  these  relations  was  sug- 
gested by  Lewis  and  McKeown.1  In  several  monomolecular 


<1921) 


W'    °'    MCC'    LeWlS   and   A'   McKeown'   Jowr-   Am€r>    Vtem.    Soc.    43,   1288 


RECENT  THEORIES  OF  CHEMICAL  ACTION       91 

reactions,  the  observed  velocity  constants  were  107  times  as  great 
as  those  calculated  on  the  basis  of  continuous  absorption  by 
the  oscillator  (electron),  with  the  refractive  index  taken  to  be 
unity.  These  discrepancies  are  referred  to  the  assumed  value 
for  the  refractive  index,  which  it  is  stated  in  this  last  article 
actually  should  refer  to  the  individual  molecules,  and  be  of  con- 
siderably greater  magnitude.  The  following  expression  was 
developed: 

8jt2  e2  n3m  v2       —  hv/kT 

k  = = — ^ — .  e  (10) 

mono  3mc3 


in  which  ^mono  represented  the  monomolecular  reaction  velocity 

constant,  e  and  m  the  charge  and  mass  of  the  electron,  v  the  fre- 
quency of  the  radiation  characteristic  of  the  reaction,  h  and  k  the 
constants  of  Planck  and  of  Boltzmann,  T  the  absolute  tempera- 
ture, and  nm  the  refractive  index  of  the  substance  in  an  ideal  state 
corresponding  to  the  closest  possible  packing  of  the  molecules. 
This  term  nm  was  shown  in  several  ways  to  have  a  value  ap- 
proximately 200,  practically  independent  of  the  system  con- 
sidered and  of  the  temperature.  This  expression  was  shown  to 
agree  satisfactorily  with  the  experimental  data  for  monomole- 
cular reactions,  so  far  as  these  were  available.  Criticisms  of 
the  radiation  theory  were  also  considered  in  this  article,  and 
especial  emphasis  placed  on  the  distinction  between  thermal  and 
photochemical  processes,  which  were  confused  in  some  of  these 
criticisms.  In  photochemical  reactions,  the  temperature  of 
radiation  is  greater  than  that  of  the  material  on  which  it  acts, 
while  in  thermal  reactions,  the  temperatures  of  the  radiation  and 
of  the  material  are  identical. 

In  discussing  certain  criticisms  of  Marcelin's  views,  Adams  x 
presented  some  relations  which  are  of  interest  because  of  their 
fundamental  character,  and  may  therefore  be  quoted:  "In  the 
last  few  years  many  attempts  have  been  made  to  apply  the 
quantum  theory  to  problems  of  chemical  dynamics.  Some  of 
these  applications  have  made  use  of  a  quantum  theory  in  a 
rather  more  definite  form  than  has  been  found  necessary  in  the 
physical  applications  where  the  theory  has  had  much  success. 

JE.  P.  Adams,  Jour.  Amer.  Chem.  Soc.  -J3,  1251   (1921). 


92  CATALYTIC  ACTION 

If  the  laws  of  chemical  dynamics  are  found  to  be  consistent  with 
the  principles  of  statistical  mechanics  it  would  seem  to  be  not 
only  unnecessary,  but  unjustifiable,  to  introduce  the  quantum 
theory  in  order  to  derive  these  laws.  That  the  quantum  theory 
is  of  importance  in  general  chemical  theory  is  probable;  neither 
statistical  mechanics  nor  the  first  two  laws  of  thermodynamics 
give  any  information  as  to  the  constants  which  enter  into  the 
various  thermodynamic  functions.  These  constants  depend 
upon  the  behavior  of  systems  at  or  near  the  absolute  zero  of 
temperature,  and  it  is  in  this  region  that  the  quantum  theory 
has  had  considerable  success.  But  this  is  quite  a  different 
matter  from  assuming  that  the  mechanism  of  chemical  reactions 
and  transformations  requires  the  use  of  the  discontinuities  in- 
herent in  any  form  of  a  quantum  theory." 

The  recent  theories  of  chemical  reactions  have  been  pre- 
sented here  in  a  somewhat  disjointed  form.  There  is  not  at 
present  an  agreement  among  the  different  workers  as  to  the  most 
probable  and  most  useful  view  to  be  adopted  with  regard  to  the 
application  of  the  radiation  phenomena,  quantum  theory,  etc. 
It  was  therefore  considered  preferable  to  present  the  views  of 
some  of  the  individuals  who  are  actively  engaged  upon  these 
problems,  even  if  these  views  contradict  each  other  in  some 
respects,  rather  than  to  attempt  a  summary  or  outline  of  the 
present  status  of  the  problem,  since  the  problem  is  continually 
presenting  new  aspects  and  developing  novel  features.  It  is  evi- 
dent that  the  theoretical  deductions  with  reference  to  the  phe- 
nomena underlying  the  kinetics  of  monomolecular  (and,  of 
course,  also  of  polymolecular)  reactions  are  not  as  yet  in  definite 
form.  The  radiation  and  quantum  theories  have  given  some 
new  and  interesting  points  of  view,  which  in  all  probability  will 
ultimately  lead  to  a  more  satisfactory  theory  with  respect  to 
many  of  the  relations. 

In  brief,  the  views  which  appear  to  have  been  generally  ac- 
cepted are  as  follows:  The  cause  of  the  monomolecular  reaction 
velocity  law  is  to  be  found  in  the  electronic  arrangements  in  the 
atoms  and  molecules  and  the  changes  in  these  arrangements  due 
to  certain  vibratory  motions;  certain  configurations  are  respon- 
sible for  the  occurrence  of  chemical  changes  (or  electronic  rear- 
rangements) ,  the  frequency  of  the  occurrence  of  these  configura- 


RECENT  THEORIES  OF  CHEMICAL  ACTION       93 

tions  obeying  some  law  of  probability;  radiant  energy  may  play 
an  important  part  in  bringing  about  certain  configurations  and 
changes  in  configurations,  but  radiant  energy  in  bringing  about 
such  configurations  and  changes  is  not  absorbed  according  to 
the  simple  Einstein  law  of  photochemical  equivalence. 

The  suggestion  made  earlier  in  this  chapter  may  be  repeated 
here,  namely,  that  a  very  limited  portion  of  the  electronic  struc- 
ture of  an  atom  or  a  molecule  is  involved  in  a  chemical  reaction, 
or  that  in  the  course  of  the  motions  of  the  electrons,  certain 
relative  positions  are  arrived  at,  the  frequency  of  the  occurrence 
of  such  positions  being  given  by  some  form  of  the  probability 
law.  These  relative  positions  either  bring  about  a  further  rear- 
rangement of  electrons  or  cause  the  first  step  of  a  series  of  such 
changes  whose  net  result  is  ultimately  called  a  chemical  reaction. 
Such  a  view  is  analogous  in  some  respects  to  the  "trigger"  action 
assumed  by  Langmuir  and  by  others. 

It  might  be  considered  that  the  next  question  to  be  taken  up 
here  would  involve  the  theories  of  the  electronic  arrangements 
and  motions  in  atoms  and  molecules.  The  Bohr  atom  may  be 
cited  as  an  illustration  of  such  a  theory.  It  would  lead  too  far, 
however,  to  go  into  these  questions  as  the  results  are  not  at  all 
conclusive  as  yet.  Only  for  the  simplest  elements  such  as  hydro- 
gen and  helium,  are  the  models  which  have  been  proposed,  at 
all  satisfactory.  Further  work  along  these  lines  will  unques- 
tionably lead  to  more  satisfactory  theories,  but  for  the  present, 
reference  to  some  of  the  published  work  is  all  that  is  possible 
here.1 

The  relation  of  the  more  recent  theories  of  chemical  actions 
to  catalytic  actions  has  Been  considered  only  incidentally  in 
this  chapter.  Any  theory  of  chemical  actions  developed  with- 
out limiting  their  nature  or  type  must  necessarily  be  applicable 
as  well  to  catalytic  actions. 

1  Cf.  J.  W.  Nicholson,  J.  Chem.  Soc.  115,  855  (1919),  "Emission  Spectra 
and  Atomic  Structure";  J.  H.  Jeans,  J.  Chem.  Soc.  115,  865  (1919),  "The  Quan- 
tum Theory  and  New  Theories  of  Atomic  Structure"  ;  I.  Langmuir,  Science  53, 
290  (1921),  "The  Structure  of  the  Static  Atom";  Physical  Review  17,  339 
(1921),  "The  Structure  of  the  Helium  Atom";  R.  A.  Millikan,  Physical  Review 
18,  456  (1921),  "Some  Facts  Bearing  on  the  Structure  of  Atoms,  Particularly 
of  the  Helium  Atom"  ;  and  others. 


Chapter  VI. 

Enzyme  Actions. 

Enzymes  are  generally  defined  as  catalysts  produced  by  living 
matter.  This  definition  replaces  the  single  term  "enzyme"  by 
the  two  terms  "catalyst"  and  "living  matter."  At  first  sight  it 
would  appear  that  the  significance  of  enzymes  might  be  ob- 
scured by  the  introduction  of  two  new  factors  which  apparently 
are  as  difficult  to  define  as  the  original  term.  It  must  be  pointed 
out,  however,  that  the  new  terms  make  possible  a  comparison 
with  phenomena  which  are  not  obviously  connected  with  the 
original  term  and  in  this  way  aid  in  the  consideration  of  enzymes 
from  various  points  of  view  and  in  relation  to  different  phe- 
nomena. The  use  of  the  definition  is  therefore  justified.  An 
attempt  will  be  made  in  this  chapter  to  review  briefly  some  of 
the  phenomena  of  enzyme  actions  especially  in  relation  to  other 
chemical  actions. 

The  chemical  changes  in  living  matter  are  similar  to  the 
chemical  changes  occurring  in  non-living  matter.  That  is  to 
say,  the  laws  governing  the  chemical  transformations  are  the 
same  for  the  phenomena  of  living  and  non-living  matter,  so  far 
as  known.  Many  of  the  substances  in  living  matter  are  of  great 
complexity.  This  complexity  alone  does  not  distinguish  such 
substances  from  substances  in  non-living  matter,  where  com- 
pounds equally  complex  are  known.  At  the  same  time  a  dif- 
ference is  seen  in  the  fact  that  many  of  the  former,  as  long  as 
they  form  part  of  the  living  matter,  undergo  change  at  a  fairly 
rapid  rate  at  moderate  temperatures.  The  changes  which  may 
take  place  in  a  given  substance  need  not  be  the  same  in  different 
forms  of  living  matter.  For  example,  a  given  protein  which  is 
used  as  food  by  different  animals  will  be  transformed  so  that  in 
each  case  the  protein  or  other  nitrogenous  substances  charac- 
teristic for  a  definite  animal  species  will  be  formed,  The  mech- 

94 


ENZYME  ACTIONS  95 

anism  of  such  changes  involves  the  breakdown  of  the  special 
protein  to  the  simpler  constituents  such  as  amino  acids  followed 
by  the  building  up  of  the  protein  or  other  substance  necessary 
for  the  animal  in  question,  and  elimination  of  material  not  re- 
quired. Analogous  changes  may  occur  with  fats,  carbohydrates, 
etc.  These  substances  are  all  of  varying  degrees  of  complexity, 
frequently  of  unknown  chemical  structure,  and  in  some  cases 
even  of  unknown  chemical  composition.  The  changes  take  place 
at  moderate  temperatures  and  under  conditions  which,  outside 
the  living  animal,  would  result  in  extremely  slow  transformations. 
Also  the  changes  in  the  living  body  occur  along  definite  lines 
producing  definite  products  from  substances  which  would  permit 
of  a  great  number  of  possible  products.  With  plants  or  living 
vegetable  matter,  the  relations  are,  in  principle,  the  same. 

Changes  similar  to  those  which  occur  in  living  matter,  can 
in  many  cases  be  brought  about  by  preparations  obtained  from 
such  matter.  The  actions  of  such  preparations  are  known  as 
enzyme  actions,  and  it  is  apparent  that  they  would  be  included 
under  catalytic  actions  with  any  of  the  ordinary  definitions  of 
the  latter.  No  attempt  is  made  in  this  preliminary  survey  to 
define  life.  The  chemical  reactions  of  living  matter  are  being 
considered,  and  these  are  characterized  by  changes  occurring  in 
definite  ways,  peculiar  in  any  one  case  to  the  system  under  in- 
vestigation. It  may  be  recalled  that  Berzelius  enumerated  such 
changes  in  one  of  his  earliest  descriptions  of  phenomena  to  be 
included  under  catalysis.1 

Enzyme  actions  may  perhaps  be  surveyed  best  from  two 
points  of  view;  first,  as  dependent  upon  the  enzyme  prepara- 
tions producing  the  actions,  and  second,  as  dependent  upon  the 
chemical  reactions  involved.  These  two  points  of  view  will 
overlap  to  some  extent. 

The  chemical  study  of  enzyme  actions  as  ordinarily  carried 
on  consists  almost  entirely  of  the  investigation  of  the  actions  of 
enzyme  preparations  on  substances  in  vitro.  While  enzyme 
studies  can  be  carried  on  to  a  certain  extent  in  vivo,  the  amount 
of  this  kind  of  work  has  been  comparatively  small,  and  although 
of  the  greatest  interest  has  been  more  in  the  nature  of  testing 
hypotheses  and  conclusions  obtained  from  the  work  in  vitro 

1  Cf.  Chapter  I,  p.  12, 


96  CATALYTIC  ACTION 

where  more  accurate  control  of  the  conditions  was  possible.  The 
evidence  for  the  presence  of  an  enzyme  in  a  preparation  is  taken 
to  be  an  increase  in  the  velocity  of  a  definite  chemical  reaction 
as  a  result  of  the  addition  of  the  preparation,  or  if  a  chemical 
reaction  can  occur  so  as  to  form  two  or  more  sets  of  products, 
a  change  in  the  relative  proportions  of  the  sets  of  products  due 
to  the  addition  of  the  preparation. 

The  chemical  reactions  which  may  be  mentioned  as  involved 
in  enzyme  actions  include  the  two  great  groups  of  hydrolysis 
reactions  and  oxidation-reduction  reactions.  Under  hydrolysis 
reactions  are  included  the  hydrolysis  of  esters  and  fats  to  form 
alcohols  and  acids;  of  complex  carbohydrates  to  form  simpler 
carbohydrates,  including  starches,  polysaccharides  such  as 
sucrose,  etc.,  to  form  finally  monosaccharides;  of  proteins  and 
their  derivatives,  to  form  simpler  bodies  and  finally  ammo- 
acids;  of  urea,  to  form  ammonia  and  carbon  dioxide;  and  of 
many  additional  classes  of  substances,  most  of  which  are  repre- 
sented by  various  derivatives  in  living  matter.  Under  oxida- 
tion-reduction reactions  are  included  the  oxidations  (and  also 
the  reductions)  which  occur  in  the  chemical  reactions  taking 
place  in  living  matter,  either  by  the  participation  of  the  oxygen 
of  the  air,  or  indirectly  by  the  oxygen  of  the  air  forming  peroxides 
with  certain  organic  substances  present  which  then  react  farther, 
or  by  inter-  or  intra-molecular  oxidation  and  reduction  without 
involving  any  substances  from  without. 

Enzyme  preparations  can  be  obtained  from  many  sources. 
By  choosing  the  material  which  in  the  living  plant  or  animal 
is  associated  with  a  certain  chemical  transformation  of  definite 
substances,  preparations  can  be  obtained  which  take  part  in 
the  same  or  analogous  reactions  in  vitro.  Thus,  pancreas  prepa- 
rations will  hydrolyze  proteins,  starches,  and  fats;  potato  prepa- 
rations will  hydrolyze  starch;  many  plant  and  tissue  prepara- 
tions will  show  oxidizing  actions;  etc. 

The  qualitative  or  descriptive  study  of  enzyme  actions  may 
proceed  from  two  points  of  view.  In  the  first  place,  attention 
may  be  fixed  upon  a  certain  chemical  reaction  and  a  search  made 
for  animal  and  vegetable  materials  such  as  tissue  extracts,  veg- 
etable extracts,  etc.,  which  give  evidence  for  the  requisite  actions. 
Secondly,  a  definite  preparation  may  be  tested  with  a  number  of 


ENZYME  ACTIONS  97 

different  chemical  reactions  in  order  to  determine  the  different 
enzyme  actions  it  possesses.  Such  studies  have  been  carried  on 
extensively.  For  the  detailed  results  reference  must  be  made 
to  the  larger  compilations  on  enzymes.  Some  of  the  actions  and 
behaviors  of  enzymes  more  pertinent  to  the  general  phenomena 
of  catalysis  and  related  to  similar  reactions  which  occur  in  the 
absence  of  enzymes  will  be  discussed  here.  An  attempt  will  be 
made  to  emphasize  the  more  quantitative  aspects  of  the  problem. 
Before  proceeding  to  those,  some  of  the  general  properties  of 
enzyme  preparations  may  be  mentioned. 

In  the  first  place,  no  enzyme  is  known  in  a  state  of  purity 
as  a  chemical  individual.  Practically  all  enzymes  exist  as  col- 
loids or  are  intimately  associated  with  substances  having  col- 
loidal properties.  The  methods  used  for  obtaining  and  purify- 
ing enzyme  preparations  are  essentially  the  same  as  those  used 
in  obtaining  preparations  from  biological  materials  in  general, 
with  the  important  reservation,  which  will  be  spoken  of  in  more 
detail  presently,  that  the  treatments  such  as  addition  of  acids 
or  alkalies  or  heating  must  be  eliminated  to  a  great  extent,  and 
also  a  number  of  reagents  must  not  be  used,  as  otherwise  the 
enzyme  property  is  lost  or  destroyed.  Water  extraction,  ex- 
traction by  neutral  salt  solutions,  or  by  glycerin  solution,  or  by 
other  mixtures,  followed  by  purification  methods  such  as  dialysis 
or  repeated  precipitations  by  salts,  alcohol,  acetone,  etc.,  may  be 
used,  each  preparation  requiring  special  study  in  order  to  deter- 
mine the  best  method  of  handling  so  that  as  much  contaminat- 
ing material  as  possible  may  be  removed  without  at  the  same 
time  causing  inactivation  of  the  enzyme. 

Every  enzyme  preparation  which  has  been  obtained  up  to 
the  present  contains  nitrogen.  The  amount  present  varies  with 
the  different  enzymes,  and  also  at  times,  for  the  same  enzyme 
property,  with  the  source  and  the  method  of  preparation.  The 
nitrogen  content  has  been  found  to  range  from  the  very  highly 
purified  sucrase  preparations1  with  1.3%,  to  the  amylase2  and 
lipase  3  preparations  which  appear  to  consist  essentially  of  pro- 
tein matter  (15%  to  17%  nitrogen).  The  nitrogen  in  all  the 

1 J.  M.  Nelson  and  S.  Born,  Jour.  Amer.  Chem.  800.  86.  393  (1914)  ;  H. 
Euler  and  O.  Svanberg,  Z.  physiol.  Chem.  112,  282  (1920). 

2  T.  B.  Osborne,  Jour.  Amer.  Chem.  Soc.  17.  587  (1895)  ;  H.  C.  Sherman  and 
M.  D.  Schlesinger,  Jour.  Amer.  Chem.  Soc.  3J,  1104  (1912),  J7,  643  (1915). 

«K.  G.  Falk  and  K.  Sugiura,  Jour.  Amer.  Chem.  Boo.  38,  921  (1916). 


98  CATALYTIC  ACTION 

preparations  studied  was  present  in  the  form  of  protein  and  in 
every  case  as  the  usual  nitrogenous  constituents  of  pro- 
teins, without  showing  greater  variations  in  these  constituents 
than  did  proteins  from  other  sources.  The  remaining  constitu- 
ents of  enzyme  preparations  showed  no  specific  characteristics, 
consisting  in  the  different  cases  of  carbohydrate  complexes,  fatty 
constituents,  inorganic  groups  and  elements,  etc.  In  fact,  it 
may  be  questioned  whether  the  chemical  compositions  of  the 
enzyme  preparations  as  well  as  the  colloidal  properties  do  not 
represent  merely  the  source  of  the  enzyme  material.  At  the 
same  time  it  must  be  stated  in  this  connection  that  attempts  to 
obtain  simpler  enzyme  bodies  from  the  complex  materials  have 
resulted  in  almost  all  cases  in  destruction  of  the  enzyme  prop- 
erties. This  is  not  the  place  to  enter  into  the  question  of  the 
chemical  structures  of  enzymes.1  The  purity  of  enzyme  prepa- 
rations, their  chemical  configurations,  the  changes  in  structure 
on  inactivation,  and  similar  questions,  are  of  secondary  impor- 
tance for  the  present  purpose.  At  the  same  time,  the  fact  must 
not  be  lost  sight  of,  that  for  a  complete  and  satisfactory  under- 
standing of  enzyme  actions,  whether  considered  as  a  group  of 
catalytic  actions  or  as  ordinary  chemical  phenomena,  a  knowl- 
edge of  the  exact  chemical  composition  and  configuration  of  the 
enzyme,  whether  a  complex  molecule  or  one  grouping  of  a  com- 
plex molecule,  is  essential. 

To  return  to  the  properties  of  enzymes,  one  of  the  most  inter- 
esting of  these  and  one  which  is  being  studied  extensively  is  the 
change  in  activity  with  change  in  hydrogen  ion  concentration.  It 
has  been  found  that  every  enzyme  action  shows  a  maximum 
action  at  a  more  or  less  definite  hydrogen  ion  concentration.  In 
some  cases  this  optimum  is  very  sharp,  in  others  it  extends  over  a 
wide  range.  A  list  of  these  has  been  given  elsewhere.2  If  the 
action  of  the  hydrogen  ion  concentration  were  limited  to  enzyme 
actions,  it  might  not  be  worth  while  to  devote  much  space  to  it 
here.  This  is  not  the  case,  however.  The  hydrogen  ion  con- 
centration of  the  medium  for  most  reactions  which  take  place  in 
the  liquid  phase,  either  in  solution,  or  between  liquids,  or  be- 
tween a  solid  or  gas  and  a  liquid,  influences  profoundly  the  reac- 

1  Cf.  "The  Chemistry  of  Enzyme  Actions,"  Chapter  VII,  for  a  review  of  the 
recent  work  on  this  question. 

a  "The  Chemistry  of  Enzyme  Actions,"  pp.  65-6. 


ENZYME  ACTIONS  99 

tions.  This  is  observed  so  strikingly  in  certain  reactions,  that 
the  hydrogen  (or  hydroxyl)  ion  has  been  assumed  to  be  the 
predominating  factor,  even  where  it  itself  is  not  changed  at  the 
end  of  the  reaction.  The  observed  effects  have  been  ascribed 
to  the  catalytic  actions  of  these  ions.  Among  the  reactions 
which  may  be  cited  are  the  hydrolysis  of  sucrose  (and  other 
carbohydrates)  by  hydrogen  ions,  the  hydrolysis  of  proteins  by 
hydrogen  or  hydroxyl  ions,  the  hydrolysis  of  esters  by  hydrogen 
or  hydroxyl  ions,  etc.  The  interest  in  connection  with  enzymes 
is  enhanced  by  the  fact  that  these  same  reactions  are  the  ones 
which  have  been  studied  most  carefully  from  the  point  of  view 
of  the  hydrogen  ion  concentration. 

The  catalytic  actions  of  acids  and  bases  (or  of  hydrogen 
and  hydroxyl  ions)  have  been  treated  in  most  text  books  as  per-  • 
haps  the  best  examples  of  catalytic  actions  which  have  been 
studied.  The  point  of  view  adopted  here  is  somewhat  different. 
Emphasis  is  placed  mainly  on  the  chemical  change  which  occurs, 
not  on  the  unchanged  factor  (the  catalyst)  in  the  reaction. 

The  hydrolysis  of  sucrose  was  one  of  the  earliest  reactions 
for  which  accurate  velocity  determinations  were  made.  In 
aqueous  solution,  the  rate  of  reaction,  as  shown  by  the  velocity 
constants,  increases  with  increase  in  hydrogen  ion  concentration. 
Quantitative  agreement  is  shown  if  the  hydrogen  ion  concentra- 
tion is  small,  but  if  it  is  high,  the  rate  increases  more  rapidly 
than  the  hydrogen  ion  concentration.  Hydroxyl  ions  do  not 
appear  to  influence  the  reaction.  The  rate  of  change  is  negli- 
gible at  moderate  temperatures  for  very  small  hydrogen  ion 
concentrations  (10-50N  or  less).  The  catalytic  action  is  only 
observed  therefore  in  solutions  which  are  ordinarily  considered 
to  be  acid.  The  action  of  the  enzyme  sucrase,  at  different  hydro- 
gen ion  concentrations  is  in  marked  contrast  to  these  actions. 
An  optimum  was  observed  at  very  nearly  [H+]  =  10'4-5  N  for 
the  sucrase  preparations  from  yeast,1  potatoes,2,  and  bananas.3 
These  preparations  were  inactive  (as  far  as  their  sucrase  actions 
were  concerned)  at  [H+]  =  lO'2  5  N  and  lO"7-0  N.  A  sucrase 
preparation  obtained  from  pneumococcus  having  an  optimum  ac- 

.  P.  L.  Sorensen,  Biochem.  Z.  21,  131   (1909)  ;  L.  Michaelis  and  H    David- 

fcftTffcftife).       ;  H' A"  Fales  and  J'  M'  Nelson' J0ttr-  Amer- 

2G.  McGuire  and  K.  G.  Falk,  J.  Gen.  Phyaiol.  2,  215   (1920). 
•G.  McGuire  and  K.  G.  Falk,  J.  Gen.  Physiol.  3,  595   (1921). 


100  CATALYTIC  ACTION 

tion  at  [H+]  =  10~7  °  N  has  been  described,1  while  the  optimum 
action  for  intestinal  sucrase  was  found  to  be  at  [H+]  =  10~6  8  N.2 
The  hydrolysis  of  sucrose  to  form  glucose  and  fructose  is  not  a 
simple  reaction.  The  complete  reaction  would  include  the  for- 
mation of  the  a  and  (3  forms  of  glucose  and  fructose  and  the 
ethylene  oxide  and  butylene  oxide  forms  of  the  latter.3  With 
acids,  there  are  probably  formed  equilibrium  mixtures  of  these 
different  forms  as  the  hydrolysis  reactions  proceed,  the  acids 
bringing  about  the  equilibria  rapidly;  with  sucrase,  the  reaction 
appears  to  proceed  only  to  the  formation  of  the  a-glucose  and 
a- fructose. 

The  quantitative  results  on  the  hydrolysis  of  proteins  by 
acids  and  alkalies  is  rather  scanty.  A  recent  study  4  of  this 
question  has,  however,  supplied  some  of  the  desired  data. 
Northrop  followed  the  hydrolysis  of  gelatine  at  hydrogen  ion 
concentrations  between  10"05  N  and  1Q-14N  at  25°  and  40°.  In 
strongly  acid  solution,  [H+]  =  10~2  °  N,  the  velocity  of  hydrolysis 
was  proportional  to  the  hydrogen  ion  concentration  as  deter- 
mined by  the  hydrogen  electrode.  In  strongly  alkaline  solution, 
[H+]  =  10"10  °  N,  the  rate  was  proportional  to  the  hydroxyl  ion 
concentration  determined  similarly.  The  hydroxyl  ions  hydro- 
lyzed  the  gelatine  30  times  as  rapidly  as  did  the  hydrogen  ions 
at  the  same  concentration.  This  should  lead  to  a  minimum  rate 
of  hydrolysis  at  about  [H+]  =  10r6-°N.  Experimentally  it  was 
found  that  a  minimum  rate  of  hydrolysis  occurred  at  this  hydro- 
gen ion  concentration,  but  that  the  actual  amount  of  hydrolysis 
was  300  times  as  great  as  that  calculated  on  the  assumption 
that  the  hydrogen  and  hydroxyl  ions  acted  to  the  same  propor- 
tionate extent  as  in  the  more  acid  and  more  alkaline  solutions. 
Since  there  is  no  reason  to  assume  that  the  hydrogen  and 
hydroxyl  ions  have  different  properties  or  behave  differently  in 
the  neighborhood  of  the  neutral  point  than  in  the  strongly  acid 
or  alkaline  solutions,  the  explanation  was  advanced  that  a 
change  in  the  gelatine  occurred  at  the  different  ranges  of  hydro- 
gen ion  concentrations.  A  comparison  of  the  deviation  of  the 

1O.  T.  Avery  and  G.  E.  Cullen,  ,J.  Exp.  Med.  32,  583   (1920). 
2H.  Euler  and  O.  Svanberg,  Z.  physiol.  Cliem.  115,  43   (1921). 
8  Cf.   "The  Chemistry  of  Enzyme  Actions,"   p.  44,  for  a   summary   of   the.sa 
relations. 

«  J.   H.  Northrop,  J.  Gen.  Physiol.   3,  715    (1921). 


ENZYME  ACTIONS  101 

hydrolysis  rate  from  that  assuming  proportionality  of  hydrogen 
and  hydroxyl  ion-  actions  throughout,  with  the  percent  of  uncom- 
bined  gelatine  present  at  the  given  hydrogen  ion  concentrations 
as  shown  by  the  titration  curve  for  gelatine,  led  to  the  conclu- 
sion that  the  uncombined  gelatine  was  hydrolyzed  about  200 
times  as  rapidly  as  the  combined  (or  ionized),  if  the  efficiency 
of  the  actions  of  the  hydrogen  and  hydroxyl  ions  was  the  same 
both  for  the  uncombined  and  combined  gelatine.  It  was  found 
that  the  hydrolysis  of  gelatine  at  constant  hydrogen  ion  concen- 
tration obeyed  the  monomolecular  reaction  velocity  law  for 
about  one-third  of  the  reaction. 

The  effect  of  the  hydrogen  ion  concentrations  on  the  proteo- 
lytic  enzymes  (proteases)  is  apparently  not  so  simple.  As  a 
matter  of  convenience,  it  has  become  customary,  especially  with 
enzyme  actions,  to  indicate  the  hydrogen  ion  concentration  by 
the  symbol  pH  which  represents  the  negative  exponent  of  10 
(or  the  negative  value  of  the  logarithm  to  the  base  10)  of  the 
number  representing  the  hydrogen  ion  concentration.  The  fol- 
lowing list  gives  a  number  of  the  values  for  the  optimum  hydro- 
gen ion  concentrations  for  the  hydrolysis  of  proteins,  usually  at 
37°,  which  are  recorded  in  the  literature. 

pH 

Pepsin   (edestin,  casein) x 1.4 

Pepsin   (egg  albumin) 2 1.4 

Pepsin  (caseinogen,  10-15  min.)3 1.8 

Pepsin  (egg  albumin,  %  to  1  hr.)4 1.6 

Pepsin  (egg  albumin,  12  hrs.)* 1.2  * 

Trypsin,  pancreatic    (albumose)5 7.7 

Trypsin,  pancreatic   (casein)6 8.3 

Trypsin,  pancreatic   (casein) 7 5.5-6.3 

Trypsin,  pancreatic   (fibrin) 7 7.5-8.3 

Erepsin,  intestinal  (albumose)8 7.7 

Protease,  yeast   (peptides)9 6.8-8.5 

1  L.  Michaelis  and  A.  Mendelssohn,  Biochem.  Z.  65,  1   (1914). 
2S.  Okada,  Biochem.  J.  10,  126   (1916). 

8L.  Michaelis  and  H.  Davidsohn,  Z.  exp.  Path.  Therap.  8,  398   (1910). 
*S.  P.  L.   Sorensen,  Biochem.  Z.  21,  131    (1909). 
BL.  Michaelis  and  H.  Davidsohn,  Biochem.  Z.  36,  280   (1911). 
«  H.  C.  Sherman  and  D.  E.  Neun,  Jour.  Amer.  Chem.  Soc.  38,  2203   (1916)  ; 
.40,  1138    (1918). 

7  J.  H.  Long  and  M.  Hull,  Jour.  Amer.  Chem.  Soc.  39,  1051   (1917). 

8  P.  Rona  and  F.  Arnheim,  Biochem.  Z.  57,  84   (1913). 

9  E.  Abderhalden  and  A.  Fodor,  Fermentforschung  1,  533   (1916). 


'i02' "  *  CATALYTIC  ACTION 

Protease,  takadiastase  (albumose)1 5.1 

Pepsin,  yeast  (proteins) 2 4.0-4.5 

Trypsin,  yeast  (peptones)2 7.0 

Erepsin,  yeast  (peptides) 2 7.8 

Pepsin,  animal  tissues  (gelatine)3 3.0-3.5 

Trypsin,  animal  tissues  (peptone)3 7.8 

Erepsin,  animal  tissues  (glycylglycine)3 7.8 

Papain  (egg  albumin,  gelatine)4 5.0 

There  is  no  regularity  apparent  in  these  results.  This  is 
not  so  surprising  when  it  is  considered  that  the  proteins  which 
were  used  as  substrates  in  most  of  this  work  were  complex 
bodies  of  unknown  compositions  and  structures,  and  that  the 
enzyme  preparations  as  well  as  the  substrates  were  present  in 
various  stages  of  purity,  or  perhaps  better  with  varying  quan- 
tities of  contaminating  materials.  Further,  the  hydrolysis  of  a 
protein  involves  decomposition  of  peptide  linkings  accompanied 
by  the  addition  of  the  atoms  of  molecules  of  water.  Peptide 
linkings  of  different  resistance  to  decomposition  exist  in  a  pro- 
tein molecule.  The  order  in  which  these  decompose  may  well 
be  different  under  different  conditions.  For  the  present,  it  ap- 
pears to  be  necessary  to  study  a  number  of  the  individual  reac- 
tions more  thoroughly  in  order  to  obtain  a  general  viewpoint 
and  explanation.  A  definite  beginning  has  been  made  by 
Northrop.5  It  was  shown  that  the  reaction  between  pepsin  and 
a  protein  really  occurs  between  ionized  protein  and  free  pepsin. 
"The  optimum  hydrogen  ion  concentration  for  the  digestion  of 
the  protein  must  coincide  with  the  hydrogen  ion  concentration 
at  which  the  concentration  of  protein  ions  and  therefore  the 
conductivity  due  to  the  protein  is  at  a  maximum."  The  pro- 
tein cations  were  obtained  from  the  protein  salt  which  was  formed 
by  adding  acid  to  protein.  After  a  certain  amount  had  been 
added,  further  addition  of  acid  would  result  in  a  decrease  in  the 
concentration  of  protein  ions  because  of  the  increase  in  concen- 
tration of  the  common  anion.  A  maximum  concentration  of 
protein  ions  would  therefore  result.  "The  limiting  pH  for  the 

S.  Okada,  Biochem.  J.  10,  130   (1916). 
K.  G.  Dernby,  Biochem.  Z.  81,  109    (1917). 
K.  G.  Dernby,  J.  Biol.  Chem.  35,  179   (1918). 
E.  M.  Frankel,  J.  Biol.  Chem.  31,  201   (1917). 

Cf.  J.   H.  Northrop,  Science,  53,  391    (1921),  for  a  summary  of  this  work 
together  with  references  to  the  articles  giving  the  experimental  details. 


ENZYME  ACTIONS  103 

activity  of  pepsin  on  the  alkaline  side  must  depend  on  the  iso- 
electric  point  of  the  protein,  since  this  is  the  point  at  which  the 
protein  first  begins  to  react  with  the  acid." 

These  conclusions  mark  a  definite  advance  in  the  study  of 
protease  actions.  Although  the  emphasis  is  placed  principally 
upon  the  protein  ion  as  such,  it  is  probable  that  the  structure  or 
configuration  of  the  protein  molecule  or  complex  which  accom- 
panies the  formation  of  the  protein  salt,  is  responsible  for  the 
chemical  action  with  pepsin  as  well  as  for  the  increased  ioniza- 
tion.  This  point  is  of  secondary  importance  at  the  present  time 
and  will  unquestionably  be  cleared  up  in  due  course.  The  rela- 
tions with  the  other  protease  actions  have  not  as  yet  been  fol- 
lowed similarly,  so  that  it  is  impossible  to  go  farther  in  this 
connection  at  present. 

Northrop  x  also  studied  the  mechanism  of  the  hydrolysis  of 
gelatine  by  acid,  alkali,  pepsin,  and  trypsin.  It  was  found  that: 
"1.  Those  linkages  which  are  most  rapidly  split  by  pepsin  or 
trypsin  are  among  the  more  resistant  to  acid  hydrolysis.  2. 
Those  linkages  which  are  hydrolyzed  by  pepsin  are  also  hydro- 
lyzed  by  trypsin.  3.  Trypsin  hydrolyzes  linkages  which  are  not 
attacked  by  pepsin.  4.  Of  the  linkages  which  are  hydrolyzed 
by  both  enzymes,  those  which  are  most  rapidly  hydrolyzed  by 
pepsin  are  only  slowly  attacked  by  trypsin.  5.  Those  linkages 
which  are  attacked  by  trypsin  or  pepsin  are  among  the  ones 
first  (most  rapidly)  hydrolyzed  by  alkali.  In  general  it  may 
be  said  that  the  course  of  the  early  stages  of  hydrolysis  of  gela- 
tine is  similar  with  alkali,  trypsin,  or  pepsin  and  quite  different 
with  acid." 

The  hydrolysis  of  starch  takes  place  in  the  presence  of  acid, 
and  increases  with  increase  in  hydrogen  ion  concentration.  The 
starch  molecule  is  very  complex,  and  in  its  decomposition  a  large 
number  of  products  may  be  formed  before  the  final  product, 
glucose,  is  obtained.  Thus,  the  following  intermediate  sub- 
stances have  been  described,  either  in  the  breaking  down  of  the 
starch  by  amylases  or  diastases  or  by  acids:-  amylodextrin, 
erythrodextrin,  achroodextrin,  maltodextrin,  maltose,  glucose. 
In  the  enzymic  decompositions  of  the  starches,  the  following 
optimum  hydrogen  ion  concentrations  have  been  recorded: 

1  J.  H.  Northrop,  J.  Gen.  Phyaiol.  4,  57  (1921). 


104  CATALYTIC  ACTION 

pH 

Amylase,  pancreatic x    7. 

malt1    4.4 

"        takadiastase x    4.8 

saliva2 6. 

"        potato3    6-7. 

"        cabbage,  carrot,  white  turnip  4 6. 

yellow  turnip  4  4.-7. 

The  complex  nature  of  the  reaction  makes  the  actual  sig- 
nificance of  these  results  somewhat  uncertain.  It  is  assumed 
by  a  number  of  chemists  that  the  enzyme  preparations  which 
hydrolyze  starch  contain  a  number  of  different  enzymes,  and 
that  each  one  of  these  enzymes  takes  part  in  only  one  step  of 
the  reactions  involved  in  the  decomposition  of  the  starch.  Evi- 
dence in  favor  of  this  view  is  seen  in  the  fact  that  different 
results  were  obtained  with  amylase  preparations  acting  upon 
soluble  starch  depending  upon  whether  the  amyloclastic  (starch 
splitting,  disappearance  of  blue  color  with  iodine)  or  saccharo- 
genic  (formation  of  substances  reducing  cupric  salts)  actions 
were  followed.5  The  latter  refers  to  the  amount  of  maltose  (and 
glucose)  produced,  and  the  former  to  the  amount  of  starch  all  of 
which  is  digested  to  a  certain  point  within  a  certain  time.  In 
view  of  the  complex  natures  of  the  substances  involved  and  the 
reactions  taking  place,  it  will  not  be  profitable  to  consider  these 
reactions  farther  in  the  present  connection. 

The  hydrolysis  of  esters  and  fats  to  form  alcohols  and  acids 
has  been  studied  extensively  from  various  points  of  view.  The 
behavior  in  solutions  of  different  hydrogen  and  hydroxyl  ion 
concentrations  is  similar  to  the  behavior  of  the  proteins.  For 
equivalent  concentrations,  the  hydroxyl  ion  exerted  1400  times 
as  much  action  for  the  cases  studied  as  the  hydrogen  ion.6  In 
passing  from  an  acid  to  an  alkaline  solution,  a  minimum  action 
would  be  observed  at  about  [H+]  =  10~6  N,  the  velocity  of  reac- 
tion being  greater  in  more  acid  and  in  more  alkaline  solutions. 

1  H.   C.   Sherman,  A.   W.   Thomas,   and  M.   E.   Baldwin,  Jour.   Amer.    Chem. 
Soc.   41,  231    (1919). 

2  R.  V.  Norris,  Biochem.  J.  7,  26,  622   (1913). 

3G.  McGuire  and  K.  G.  Falk,  J.  Gen.  Physiol.  2,  215   (1920). 
*K.  G.  Falk,  G.  McGuire,  and  E.  Blount,  J.  Biol.  Chem.  38,  229   (1919). 
6  H.   C.   Sherman   and   M.   D.   Schlesinger,  Jour.   Amer.    Chem.  Soc.   35,  1784 
(1913). 

8  J.  J.  A.  Wijs,  Z.  physik.  Chem.  11,  492  ;  n,  514  (1893). 


ENZYME  ACTIONS  105 

The  velocities  run  parallel  to  the  increase  in  hydrogen  and 
hydroxyl  ion  concentrations,  but,  except  for  limited  ranges,  are 
not  strictly  proportional.  Considerable  work  has  been  done  on 
the  mechanism  of  these  reactions,  and  much  evidence  brought 
forward  to  prove  the  existence  of  intermediate  compounds.1 
The  unstable  nature  of  these  intermediate  products  and  the  con- 
sequent difficulty  attending  their  isolation,  have  obscured  the 
course  of  the  reaction.  On  the  other  hand,  the  work  of  Stieglitz 
on  the  imido  esters,  to  which  reference  has  been  made  before 
in  this  book,  has  thrown  much  light  on  the  mechanism  of  such 
reactions.  The  hydrolysis  of  the  imido  esters  by  the  addition 
of  acids  was  shown  to  be  due  to  the  increased  concentration  of 
one  of  the  reacting  species  (ester  ion  in  this  case)  and  the  rela- 
tions were  interpreted  according  to  well-known  chemical  laws. 
The  compounds  formed  in  the  different  stages  of  the  reactions 
were  isolated  and  identified,  and  the  reactions,  as  well  as  a  num- 
ber of  analogous  reactions,  were  removed  from  the  mysterious 
realms  of  catalytic  actions  and  interpreted  according  to  simple 
chemical  principles.  Imido  esters  were  studied  in  place  of  ordi- 
nary esters  because  of  the  greater  ease  of  following  the  reactions 
experimentally,  but  the  general  conclusion  arrived  at,  that  the 
increased  reaction  velocity  was  due  to  the  increased  concentra- 
tion of  the  reacting  constituents  is  unquestionably  true  for  the 
increased  hydrolysis  of  esters  because  of  the  presence  of  hydro- 
gen and  hydroxyl  ions,  and  the  natures  of  the  intermediate  com- 
pounds in  the  two  series  are  probably  similar. 

The  optimum  hydrogen  ion  concentrations  for  the  enzymic 
hydrolysis  of  esters  and  fats  have  been  determined  in  a  few 
cases.  The  results  were  as  follows: 

pH 

Esterase,  pancreatic  2 8.3-9. 

Esterase,  blood  serum  2 8. 

Lipase,  duodenal  juice  3 8.5 

Lipase,  gastric  juice  3 4.  -5. 

1Ct.  K.  G.  Falk  and  J.  M.  Nelson,  Jour.  Amer.  Chem.  Soc.  37,  1732  (1915), 
for  references  to  the  experimental  evidence  regarding  the  existence  of  these 
compounds;  also  G.  Baume  and  G.-P.  Pamfil,  J.  chim.  phys.  12,  260  (1914)  ;  J. 
Kendall  and  co-workers,  Jour.  Amer.  Chem.  S'oc.  36,  1222,  1722,  2498  (1914)  ; 
31,  149  (1915)  ;  43,  1426  (1921). 

2P.  Rona  and  Z.  Bien,  Biochem.  Z.   59,  100;   &h,  13    (1914). 

8  H.  Davidsohn,  Biochem  Z.  49,  249   (1913). 


106  CATALYTIC  ACTION 

The  reactions  here  are  simpler  than  with  the  hydrolysis  of 
the  proteins.  The  chemical  compositions  and  structures  of  the 
initial  and  final  products  of  the  reaction  (that  is  to  say,  of  the 
substances  which  are  changed)  are  known.  The  difficulty  of 
studying  this  problem  is  of  a  different  sort.  The  enzymic  ac- 
tivity of  esterase  and  lipase  preparations  is  lost  comparatively 
rapidly  as  a  rule,  even  on  simple  treatments.  This  makes  it 
difficult  to  prepare  highly  active  preparations  and  to  remove 
inactive  materials.  The  interesting  result  of  the  determinations 
of  optimum  hydrogen  ion  concentration  conditions  here,  as  with 
the  protease  preparations,  is  to  be  found  in  the  fact  that  the 
maximum  actions  in  the  presence  of  enzymes  were  obtained  at 
the  hydrogen  ion  concentration  where,  in  the  absence  of  the 
enzymes,  the  actions  approached  a  minimum. 

The  results  involving  the  effects  of  different  hydrogen  ion 
concentrations  on  the  velocity  of  these  chemical  reactions  in  the 
absence  and  in  the  presence  of  enzymes,  appear  to  indicate  that 
the  reactions  in  the  two  series  are  fundamentally  different.  If 
attention  is  fixed  upon  the  actual  chemical  change  taking  place, 
there  is  no  fundamental  difference  observable,  although  apparent 
differences  are  seen  because  a  reaction  may  have  proceeded  far- 
ther (or  several  successive  stages  may  have  occurred)  in  one  case 
than  in  another.  The  hydrogen  ion  (or  the  hydroxyl  ion)  and 
the  enzyme  are  present  after  the  reaction,  presumably  as  far  as 
can  be  told,  as  the  same  compound  as  before  (some  secondary 
changes  will  be  spoken  of  presently).  Either  from  this  point  of 
view,  or  from  the  point  of  view  of  change  in  reaction  velocity, 
both  the  hydrogen  ion  (or  hydroxyl  ion)  and  the  enzyme  would 
be  grouped  as  catalysts,  and  the  reactions  included  under  cata- 
lytic changes.  The  evidence  available  in  the  few  cases  where 
quantitative  studies  have  been  carried  out,  indicates  that  the 
course  of  the  changes  includes  the  formation  of  intermediate 
compounds  or  that  the  reactions  take  place  in  stages.  Evidence 
for,  and  deductions  from,  this  general  view  of  the  mechanism  of 
chemical  reactions,  has  been  presented  elsewhere  x  in  some  detail. 

The  relations  existing  between  the  hydrogen  ion  concentra- 
tion of  the  medium,  whether  or  not  enzyme  is  present,  and  the 
velocity  of  the  corresponding  chemical  reaction  are  so  compli- 

1  "Chemical  Reactions;  Their  Theory  and  Mechanism." 


ENZYME  ACTIONS  10? 

cated,  or  at  any  rate  have  not  been  explained  in  many  cases, 
that  it  does  not  appear  advisable  to  devote  much  space  to  con- 
ditions under  which  these  reactions  take  place  where,  in  addition, 
certain  substances  are  added  which  influence  the  velocities.  The 
general  viewpoint  or  theoretical  significance  of  these  relations 
is  unsatisfactory,  and  it  will  only  be  possible  to  group  them 
in  a  more  or  less  superficial  manner.  With  regard  to  the  reac- 
tions taking  place  in  the  absence  of  enzymes,  it  has  been  found 
that  neutral  salts  change  the  velocities  with  highly  ionized  acids, 
in  ways  which  cannot  be  accounted  for  on  the  basis  of  de- 
creased ionization  of  the  acids.  The  hydrolysis  of  sucrose  and 
of  esters  has  been  studied  in  this  way.  The  view  called  the 
"Dual  Theory  of  Catalysis"  was  developed,  in  which  the  union- 
ized acid  molecule  as  well  as  the  hydrogen  ion  was  assumed  to  act 
as  catalyst.1  The  next  step  which  was  taken  considered  ioniza- 
tion as  secondary  to  some  change  in  the  medium  involving  pri- 
marily the  solvent.  According  to  this  view,  chemical  action  and 
ionization  represented  chemical  and  physical  evidences,  which 
paralleled  each  other  in  some  cases  and  not  in  others,  of  this  more 
deep  seated  relation.2  It  may  be  stated,  however,  that  no  theory 
of  these  phenomena  has  been  generally  accepted  up  to  the  present 
time,  although  their  importance  in  catalytic  reactions  is  univer- 
sally recognized. 

It  is  obvious  that  with  enzyme  actions  the  phenomena  are 
still  more  complicated.  The  behaviors  of  the  enzyme  prepara- 
tions and  the  changes  these  may  undergo,  entirely  aside  from 
the  influence  on  chemical  reactions  upon  which  they  may  be 
said  to  exert  catalytic  actions,  must  be  considered  carefully. 
In  a  sense,  it  is  impossible  to  separate  the  properties  of  enzymes 
from  the  chemical  reactions  which  they  influence,  since  enzyme 
actions  are  known  only  because  of  these  influences. 

In  the  first  place,  enzyme  preparations  lose  their  activity 
under  comparatively  simple  conditions,  a  result  which  at  times 

1  Cf.  the  summary  in  W.  C.  McC.  Lewis,  "A  System  of  Physical  Chemistry," 
1918,  Part  I,  pp.  423-9.     The  historical  development  of  the  views  is  of  interest, 
but   can    only    be   referred   to    here.     For    example,    the   first    suggestion    of    the 
possible  activity  of  unionized  molecules  in  catalytic  actions  by  H.  Goldschmidt 
in  1899   (Z.  jthysik.   Chem.  19,  118)  ;   the  increasing  importance  ascribed  to  the 
simultaneous  actions  of  unionized  molecules  and  of  hydrogen  ions  by  J.  Stieglitz 
(Am.  Chem.  J.  39,  167   (1908)),  S.  F.  Acree  and  J.  M.  Johnson    (Am.  Chem.  J. 
31,  410   (1907)  ;  38,  329   (1908)),  and  others,  etc.,  may  be  mentioned. 

2  K.   G.   Falk  and  J.   M.   Nelson,  Jour.   Amer.   Chem.   Soc.   37,   1732    (1915)  ; 
K.  G.  Falk,  "The  Chemistry  of  Enzyme  Actions,"  pp.  48-53. 


108  CATALYTIC  ACTION 

brings  out  the  similarity  between  enzyme  actions  and  life  proc- 
esses in  a  striking  manner.  When  enzyme  preparations  are  heated 
in  solution,  at  first  the  enzyme  actions  are  increased  (as  with  all 
chemical  reactions)  up  to  a  maximum  generally  in  the  neighbor- 
hood of  38°,  and  then,  at  higher  temperatures,  the  activities  are 
lost.  This  is  the  general  phenomenon  as  observed,  but  it  is  best 
interpreted  by  considering  that  loss  of  activity  occurs  even  at 
moderate  temperatures,  that  there  is  a  balance  between  this  loss 
and  the  increased  activity  due  to  increased  temperature,  that  a 
maximum  activity  is  obtained  at  a  certain  temperature,  and  that 
finally  the  loss  predominates  until  inactivation  is  complete. 
The  conditions  of  the  environment,  such  as  acidity  of  medium, 
etc.,  are  also  involved  in  the  rate  of  inactivation  by  tempera- 
ture. Some  enzymes  are  more  resistant  to  heat  than  others,  so 
that  no  general  statement  with  regard  to  this  phenomenon  is 
possible,  except  that  the  inactivation  is  connected  with  a  chemi- 
cal change  involving  the  enzymic  molecular  structure  or 
grouping. 

The  differences  in  activity  of  enzyme  preparations  at  dif- 
ferent hydrogen  ion  concentrations  is  a  striking  feature  of  enzyme 
actions.  It  is  necessary  apparently  for  the  enzyme  molecule  or 
grouping  to  possess  a  definite  state,  configuration,  or  structure 
in  order  to  show  the  requisite  behavior.  Another  factor  is  in- 
volved, however,  but  can  be  disposed  of  briefly.  The  activity 
of  all  enzyme  preparations  can  be  destroyed  permanently  by 
treatment  with  acid  or  alkali  of  sufficient  strength.  The  con- 
centration of  hydrogen  or  hydroxyl  ions  necessary  for  this  irre- 
versible inactivation  varies  with  the  enzyme,  with  accompany- 
ing inactive  material,  with  temperature,  and  with  time  of  action. 
Since  so  many  factors  may  be  involved,  it  will  not  be  advisable 
to  enter  into  a  detailed  discussion  of  this  question  here  except 
to  point  out  that  the  change  is  unquestionably  one  involving 
change  in  internal  structure  of  the  active  enzyme  molecule  or 
grouping,  because  of  the  very  mild  acid  or  alkaline  treatments 
which  frequently  bring  about  this  inactivation.  The  reversible 
partial  inactivation  may  also  be  due  to  change  in  internal  struc- 
ture of  the  enzyme  molecule  or  grouping.  Attempts  have  been 
made  to  determine  whether  the  activity  was  connected  with  or 
part  of  the  unionized  molecule  or  one  of  the  ions.  Experiments 


ENZYME  ACTIONS  109 

showing  the  direction  of  migration  in  solution  in  an  electric  field 
have  answered  this  question  for  a  number  of  enzymes.  Michaelis 1 
gave  the  following  summary  of  the  results  which  had  been  ob- 
tained: with  sucrase,  the  unionized  molecule  contained  the  active 
enzyme;  with  trypsin,  erepsin,  lipase,  and  maltase,  the  anions 
were  active.  With  pepsin,  the  cations  appeared  to  be  active, 
but  further  careful  work  showed  that  with  a  preparation  of 
purified  pepsin  no  migration  in  an  electric  field  occurred,  and 
only  when  a  protein  (albumin  or  albumose)  was  added  did 
migration  take  place  with  the  protein.2  The  reversible  inacti- 
vation  of  enzymes  is  probably  connected  with  reversible  changes, 
perhaps  of  a  tautomeric  nature,  within  the  molecule  containing 
the  enzyme,  and  the  ionization  properties  parallel  these  changes 
in  some  cases  without  being  responsible  directly  for  the  enzyme 
actions.  It  must  also  be  recalled  that  the  differences  in  hydrogen 
ion  concentrations  may  affect  the  structure  of  the  substrate  as 
well  as  of  the  enzyme.  This  was  shown  in  the  action  of  protease 
by  Northrop,  who  found  that  it  was  necessary  for  the  protein 
which  was  acted  upon  by  pepsin  to  be  in  an  ionized  state,  and 
by  Long  and  Hull,3  who  stated  that  the  optimum  hydrogen  ion 
concentration  for  the  action  of  pancreatic  trypsin  on  casein  was 
given  by  the  value  of  pH  5.5-6.3,  and  on  fibrin  of  7.5-8.3.  It  is 
possible  that  this  view  is  of  much  wider  applicability,  that  in 
addition  to  the  condition  of  the  substrate  in  enzyme  actions  being 
one  of  the  determining  factors,  it  is  also  necessary  for  the  sub- 
stances which  are  transformed  to  possess  certain  definite  struc- 
tures in  chemical  reactions  involving  catalysts  not  enzymes.  In 
a  sense,  this  is  a  generalized  statement  of  the  phenomena  de- 
scribed by  Stieglitz  in  his  imido  ester  work  where  either  the 
formation  of  a  new  substance,  or  increasing  the  concentration  of 
a  substance  present  only  in  minimal  quantities  before  the  addi- 
tion of  the  so-called  catalyst,  resulted  in  the  speeding  up  of  the 
chemical  change.  The  imido  ester  ion  in  the  work  of  Stieglitz 
was  the  main  reacting  substance  after  the  addition  of  acid  to 
imido  ester.  The  compositions  and  structures  of  these  sub- 
stances are  known  and  are  comparatively  simple.  At  the  same 
time,  there  appears  to  be  no  reason  why  this  view  should  not  be 

JL.  Michaelis,  Biochem.  Z.  60,  91   (1914). 

2  C.  A.  Pekelharing  and  W.  E.  Ringer,  Z.  physiol.  Cheni,  75.  282   (1911). 

*  J.  H.  Long  and  M.  Hull,  Jour.  Amer.  Chem.  Soc.  39,  1051  (1917). 


110  CATALYTIC  ACTION 

extended  to  more  complex  substances  and  also  to  substances  or 
mixtures  where  the  various  intermediate  products  are  too  un- 
stable to  have  been  isolated. 

Neutral  salts  increase  the  velocity  of  hydrolysis  of  sucrose 
and  of  esters  by  highly  ionized  acids.  This  is  explained  on  the 
basis  of  the  Dual  Theory  of  Catalysis  by  considering  that  the 
neutral  salt  decreased  the  degree  of  ionization  of  the  acid  and 
since  the  acid  molecule  exerted  a  greater  action  than  the  hydro- 
gen ion,  the  velocity  of  the  reaction  was  increased.  This  view 
depends  upon  certain  assumptions  relative  to  the  degrees  of 
ionization  of  the  electrolytes.  There  has  been  considerable 
work  done,  especially  in  recent  years,  on  this  question,  and  a 
number  of  papers  published  in  which  the  view  was  advocated 
that  for  strong  electrolytes  ionization  was  complete,  and  that  the 
observed  and  calculated  values  obtained  for  the  percentages  of 
ionization  show  deviations  from  the  values  for  complete  ioniza- 
tion because  of  secondary  relations.1  This  view,  if  correct, 
obviously  would  require  modification  of  the  Dual  Theory  of 
Catalysis.  At  the  same  time,  it  may  be  pointed  out  that  on 
the  addition  theory  of  chemical  reactions,  as  presented  in  an- 
other connection,2  ionization  is  a  secondary  phenomenon,  and 
the  reactions  are  accounted  for  on  the  basis  of  the  formation  of 
intermediate  addition  compounds. 

The  actions  of  neutral  salts  on  enzymes  may  be  extended  to 
include  the  actions  of  a  large  number  of  organic  as  well  as 
inorganic  substances  which  in  one  way  or  another  modify  the 
rate  of  change  of  the  reaction  in  which  the  enzyme  is  involved. 
The  general  problem  was  seen  to  be  in  an  uncertain  state  in 
considering  the  reactions  in  the  absence  of  enzymes  and  in  the 
presence  of  such  a  chemically  simple  catalyst  as  the  hydrogen 
ion.  There  is  no  explanation  of  the  observed  facts  which  has 
as  yet  found  general  acceptance.  It  is  obvious,  then,  that  in 
enzyme  actions,  where  the  possibility  exists  of  the  neutral  salt 

1  W.  Sutherland,  Phil.  Mag.  (6)  1^,  3  (1907)  ;  S.  R.  Milner,  Phil.  Mag.  (6) 
S5,  214,  354  (1918)  ;  J.  C.  Ghosh,  J.  Chem.  Soc.  113,  449,  627  (1918)  •  N  Bier- 
rum,  Z.  Elektrochem.  W,  321  (1918)  ;  A.  A.  Noyes  and  D.  Maclnnes,  Pro'c.  Nat. 
Acad.  Sci.  6,  18  (1920)  ;  Jour.  Amer.  Chem.  Soc.  42,  239  (1920)  :  J.  C  Ghosh 
Z.  physik.  Chem.  98,  211  (1921)  ;  G.  Akerlof,  Z.  physik.  Chem.  98,  260  (1921)  ; 
H.  Kallman  Z -Physik  Chem.  98,  433  (1921)  ;  R.  H.  Clark,  Jour.  Amer.  Chem. 
Soc.  43,  1759  (1921)  ;  Lr.  Ebert,  Jahrb.  Radioaktiv.  Eletronik  18.  134  (1921) 

a  "The  Chemistry  of  Enzyme  Actions,"  pp.  49,  51. 


ENZYME  ACTIONS  111 

or  other  added  substance  acting,  not  only  upon  the  chemical 
change  which  is  being  followed,  but  also  upon  an  enzyme  sub- 
stance of  unknown  complexity  and  composition  as  a  rule,  but 
readily  modified  by  external  agencies,  the  theoretical  interpreta- 
tion of  the  experimental  facts  observed  will  be  still  less  satis- 
factory. As  indicated,  an  added  substance  may  produce  an 
effect  by  acting  upon  the  substrate  or  upon  the  enzyme.  It  has 
been  found  that  many  substances  produce  such  actions,  a  num- 
ber increasing  the  velocities  of  the  enzyme  actions,  others  de- 
creasing the  velocities.  A  considerable  literature  has  grown  up 
around  these  phenomena,  such  names  as  activators,  co-enzymes, 
anti-enzymes,  etc.,  being  used  to  denote  the  substances  or  mix- 
tures producing  these  actions.  Regularities  have  been  observed 
with  certain  series  of  salts  on  various  enzyme  actions.  The 
whole  question  is  at  present  at  the  empirical  stage.  Much  of 
the  data  is  not  useful,  because  conditions  of  experimentation 
were  not  sufficiently  controlled.  Thus,  the  hydrogen  ion  con- 
centration of  a  mixture  must  be  kept  constant  if  a  series  of  neu- 
tral salts  is  studied;  the  formation  of  precipitates  must  be  con- 
sidered in  carrying  out  enzyme  tests;  etc.  Altogether,  it  is  im- 
possible at  present  to  give  a  general  viewpoint  in  connection  with 
these  actions  and  this  is  not  the  place  to  enumerate  the  many 
changes  which  have  been  observed  and  recorded.  One  line  of 
work  may,  however,  be  mentioned.  Certain  substances  exert 
such  unique  effects  upon  the  velocities  of  definite  enzyme 
actions  that  it  appears  as  if  these  effects  would  prove  to  be  the 
most  direct  ways  of  obtaining  knowledge  of  the  changes  in- 
volved, of  the  conditions  which  make  such  changes  possible,  and 
possibly  of  the  chemical  nature  of  the  enzymes  themselves.  For 
example,  reference  may  be  made  to  the  large  increases  in  veloc- 
ities produced  by  hydrogen  cyanide  on  the  proteolytic  enzyme 
papain,1  by  bromide  (as  distinct  from  chloride  and  iodide)  on 
amylase,2  by  manganous  sulfate  on  castor  bean  lipase,3  etc. 

Before  leaving  this  part  of  the  subject,  it  may  be  mentioned 
that,  in  addition  to  the  reactions  cited  because  of  their  impor- 

!E.  M.  Frankel,  J.  BioL  Chem.  31,  201  (1917)  ;  cf.  also  H.  S.  Vines,  Ann. 
Bot.  17,  606  (1903)  ;  L.  B.  Mendel  and  A.  F.  Blood,  J.  Biol.  Chem.  8,  177  (1910) 

2  A.  W.  Thomas,  Jour.  Amer.  Chem.  Soc.  39,  1501   (1917). 

8E.  Hoyer,  Z.  physiol.  Chem.  50,  414  (1907)  ;  Y.  Tanaka,  Orig.  Com.  8th 
Intern.  Congr.  Appl.  Chem.  11,  37  (1912)  ;  K.  G.  Falk  and  M.  L.  Hamlin,  Jour 
Amer.  Chem.  Soc.  35,  210  (1913). 


112  CATALYTIC  ACTION 

tance  in  enzyme  actions,  the  velocities  of  many  chemical  reac- 
tions may  be  greatly  increased  by  the  addition  of  more  than  one 
"catalyst."  Thus  one  of  the  most  interesting  examples  which  has 
attained  considerable  importance  in  recent  years  involves  the  oxi- 
dation of  carbon  monoxide.  At  ordinary  temperatures,  this  reac- 
tion is  extremely  slow,  if  it  can  be  said  to  take  place  at  all.  The 
study  of  a  number  of  oxides  showed  that  in  the  presence  of  a 
mixture  of  the  four  oxides,  Mn02  (50%),  CaO  (30%),  Co203 
(15%),  and  Ag20  (5%),  carbon  monoxide  was  oxidized  rapidly 
by  the  oxygen  of  the  air  at  ordinary  temperatures.1  Further 
examples  of  such  actions  will  be  given  in  Chapter  VIII,  but  it 
may  be  stated  here  that  satisfactory  explanations  or  reasons  for 
the  greater  actions  of  mixtures  of  several  substances  as  com- 
pared with  the  action  of  a  single  substance,  are  lacking. 

One  of  the  most  interesting  features  of  enzyme  actions  which 
has  frequently  been  referred  to  as  characteristic  of  such  actions 
is  the  specificity.  A  certain  enzyme  preparation  acts  upon  one 
reaction  or  group  of  reactions  and  not  upon  other  reactions.  The 
enzyme  sucrase  hydrolyzes  sucrose  (and  probably  raffinose)  and 
not  maltose ;  maltase  hydrolyzes  maltose  and  not  sucrose ;  castor 
bean  esterase  and  lipase  preparations  both  hydrolyze  simple 
esters  and  glycerides,  but  to  different  extents;  the  various 
proteases  are  limited  in  their  actions  upon  proteins,  but  the  dif- 
ferent ones,  pepsin,  trypsin,  and  erepsin,  hydrolyze  proteins  or 
their  partial  decomposition  products  to  different  extents  and 
under  different  conditions;  urease  hydrolyzes  urea  rapidly  and 
methyl  urea  only  slightly  if  at  all;  etc.  This  question  was  dis- 
cussed in  some  detail  in  another  connection.2  "These  specificities 
are  striking  in  many  cases,  but  not  unique  considered  as  chemi- 
cal phenomena.  The  most  obvious  reactions  in  which  specifici- 
ties are  used  are  those  included  in  Qualitative  Chemical  Analysis 
(and  also  Quantitative  Analysis).  In  the  reactions  involving 
the  identification  of  the  metallic  elements,  these  may  be  com- 
pared to  the  substrates  in  enzyme  actions,  and  the  reagents  used 
to  enzyme  preparations  or  materials.  There  are,  in  both  cases, 

1  A.  B.  Lamb,  W.  C.  Bray  and  J.  C.  W.  Frazer,  ,7.  Ind.  Eng.  Chem.  12,  213 

2  "The   Chemistry   of  Enzyme  Actions,"   pp.   127-8.     An  example  of  such   an 
action  was  published  recently  by  B.  Abderhalden  and   H.   Handovsky    (Ferment- 
forschung   ],,   316    (1921)).     They   found    that    yeast   juice   hydrolyzed    glycyl-J- 
eucylglycyl-Heucine  but  not  glycyl-d-leucylglycyl-Heucine. 


ENZYME  ACTIONS  113 

group  reagents  and  individual  reagents.  With  enzymes,  for  ex- 
ample, amylase,  different  proteases,  emulsin,  lipase,  etc.,  act 
upon  certain  groups  of  substances.  Within  each  group  there 
will  be  smaller  differences  for  each  individual  substrate  with  the 
group  reagent.  The  conditions  must  also  be  kept  within  certain 
limits.  In  qualitative  analysis,  similarly,  hydrogen  sulfide  might 
be  used  as  an  example  of  a  reagent  showing  group  reactions  with 
certain  metallic  elements  in  solution,  as  well  as  differences  with 
the  individuals  in  the  group,  while  the  conditions  of  the  reaction 
(such  as  acidity  or  alkalinity,  etc.)  must  be  kept  within  certain 
limits.  These  analogies  might  be  multiplied  indefinitely.  One 
set  of  phenomena  is  as  remarkable  as  the  other,  but  familiarity 
with  the  one  has  made  these  reactions  commonplace,  while  the 
practical  necessity  for  replacing  definite  chemical  substances  by 
subsfances  as  yet  not  as  well  characterized  and  therefore  known 
by  names  less  familiar  has  resulted  in  enzyme  actions  and  their 
specificities  acquiring  a  certain  air  of  mystery.  This  is  unjus- 
tified, and  their  reactions  are  no  more  mysterious  than  are  other 
chemical  reactions." 

Of  similar  tenure  is  the  conclusion  of  Northrop,  who  stated, 
as  a  result  of  the  study  of  the  comparative  hydrolysis  of  gelatin 
by  pepsin,  trypsin,  acid,  and  alkali,  to  which  reference  was  made 
earlier  in  this  chapter  that  "There  does  not  seem  to  be  any 
evidence  to  distinguish  qualitatively  between  the  specificity  of 
an  enzyme  and  of  hydrogen  ions."  x 

The  substances  upon  which  many  enzymes  exert  their  actions 
are  frequently  of  complex  nature.  The  products  formed  partake 
often  of  some  of  the  characteristics  of  the  initial  substrates. 
Since  action  occurs  between  enzyme  and  substrate,  it  is  not  sur- 
prising that  action  may  also  occur  between  enzyme  and  prod- 
ucts. The  former  manifests  itself  by  an  increase  in  the  velocity 
of  the  chemical  reaction,  the  latter  by  a  modification  of  this 
increase.  Michaelis  and  Menten 2  showed  that  the  action  of 
sucrase  was  inhibited  by  the  products  of  the  reaction  it  influ- 
enced. Fructose  retarded  the  actions  much  more  markedly  than 
did  glucose.  These  facts  have  been  confirmed  and  extended  in 
various  directions  in  recent  years  by  J.  M.  Nelson  and  his  co- 

*J.  H.  Northrop,  /.  Gen.  Pbysiol.  4,  57   (1921). 

*Jv.  Michaelis  and  M.  I,.  Menten,  Biochern,.  Z.  1,9,  333   (1913). 


114  CATALYTIC  ACTION 

workers.  The  explanation  advanced  for  this  retardation  is  based 
upon  the  formation  of  chemical  compounds  of  the  sucrase  with 
glucose  and  fructose.  In  this  way  the  sucrase  is  removed  from 
the  sphere  of  action  and  is  unable  to  react  farther  with  the 
sucrose.  The  greater  retardation  shown  by  the  fructose  in  com- 
parison with  the  glucose,  points  to  chemical  phenomena  as  in- 
volved in  the  combinations.  Similar  retardations  of  enzyme 
actions  by  the  products  of  the  decomposition  of  the  substrate 
have  been  observed  in  the  action  of  amylase  on  starch  in  which 
maltose  exerted  a  retarding  influence  *  and  in  the  action  of  pepsin 
on  protein  in  which  peptone  or  albumose  caused  the  retardation.2 
A  somewhat  different  type  is  the  irreversible  inhibiting  action 
of  the  products  of  an  enzyme  action  as  shown  in  the  action  of 
benzaldehyde  on  emulsin  and  probably  also  of  simple  alcohols 
(not  glycerin)  on  esterases.3  These  phenomena  are  evidently 
special  cases  of  the  retarding  actions  of  certain  substances  on 
enzyme  actions,  simpler  in  the  sense  that  no  foreign  substance 
is  added  initially.  They  are  explained,  as  stated,  by  those  who 
have  studied  the  reactions,  as  due  to  combination  of  one  or  more 
of  the  products  with  the  enzyme  substance,  in  that  way  removing 
the  enzyme  from  the  sphere  of  action.  In  some  cases,  the  com- 
bination is  followed  by  precipitation.  Such  actions  as  change 
in  hydrogen  ion  concentration  of  the  medium  by  the  products 
of  the  reaction  are  not  directly  included  in  these  relations,  but 
even  these  may  be  considered  from  the  same  point  of  view,  the 
acid  or  alkali  produced  combining  with  the  enzyme.  In  this 
way,  the  hydrogen  ion  concentration  of  the  solution  may  well 
be  changed,  this  change  being  accompanied  by  a  change  in 
internal  structure  of  the  enzyme  molecule.  The  three  phe- 
nomena brought  about  by  acid  or  alkali;  combination  with  the 
acid  or  alkali,  change  in  structure  of  the  enzyme  grouping  or 
molecule,  and  change  in  activity,  appear  to  run  parallel. 

The  question  of  the  mathematical  formulation  of  the  kinetics 
of  enzyme  actions  may  be  considered  briefly.4  It  has  been 
pointed  out  that  the  mathematical  formulation  of  the  velocities 

*A.  Wohl  and  E.  Glimm,  BiocJiem.  Z.  27,  349  (1910)  ;  G.  McGuire  and  K.  G. 
Falk,  J.  Gen.  Physiol.  2,  224  (1520). 

2J.  H.  Northrop,  J.   Gen.  Physiol.  2,  471    (1920). 

3K.  G.  Falk,  Jour.  Amer.  Chem.  Soc.  35,  616   (1913). 

*  A  summary  of  some  of  these  relations,  especially  as  bearing  upon  the 
question  of  pepsin  action,  was  given  by  J.  H.  Northrop,  Science  53,  391  (1921"), 


ENZYME  ACTIONS  115 

of  chemical  reactions  and  changes  in  these  velocities  due  to 
added  substances  has  not  proven  satisfactory,  and  that  various 
attempts  have  been  and  are  being  made  to  reconstruct  the 
hypotheses  upon  which  the  formulations  are  based.  It  is  evi- 
dent that  the  introduction  of  another  factor,  such  as  the  enzyme, 
may  increase  the  difficulties  involved  in  the  formulations.  These 
increased  difficulties  will  not  be  gone  into  here;  but,  on  the 
other  hand,  some  of  the  properties  peculiar  to  enzymes  and 
their  actions  have  made  it  possible  to  indicate  and  make  prob- 
able from  a  somewhat  different  angle  a  possible  solution  of  some 
of  the  questions  involved. 

In  an  enzyme  action,  the  rate  of  reaction  is  found  to  be 
proportional  to  the  enzyme  concentration  in  some  cases  but 
not  in  others.  This  proportionality  can  only  be  said  to  hold 
within  certain  limits  of  concentration  of  enzyme  substance  and 
substrate.  The  interference  due  to  the  products  of  the  reaction, 
the  presence  of  other  substances,  the  necessity  in  many  cases  of 
having  both  the  enzyme  and  substrate  in  a  given  condition  as 
evidenced  by  the  hydrogen  ion  concentration  optimum,  all  may 
play  a  part  here.  Also,  the  fact  that  the  reactions  in  all  prob- 
ability take  place  in  several  stages,  as  will  be  elaborated  pres- 
ently, undoubtedly  complicates  the  question.  It  has  also  been 
found  experimentally  that  with  a  number  of  enzymes,  the  amount 
of  action  is  proportional  to  the  square  root  of  the  enzyme  con- 
centration (Schiitz's  rule). 

With  regard  to  the  change  in  substrate  concentration,  with  a 
given  enzyme  concentration  and  dilute  substrate  concentration 
the  action  is  proportional  to  the  substrate  concentration  as  a 
rule.  With  increasing  substrate  concentration,  the  increase  in 
action  is  less  rapid,  and  with  a  number  of  enzymes,  after  a  cer- 
tain concentration  has  been  reached,  the  action  is  not  increased 
by  more  substrate.  In  other  words,  there  appears  to  be  a 
maximum  capacity  of  an  enzyme  preparation  for  hydrolyzing 
a  substrate,  no  matter  how  much  excess  substrate  may  be  pres- 
ent above  a  certain  quantity.  This  relation  has  been  found  to 
hold  for  the  following  enzyme  actions:  Sucrase,  sucrose;1 

*A.  Brown,  J.  Chem.  Soc.  81,  373  (1902)  ;  E.  Duclaux,  Traite  de  Microbi- 
ologie,  Tome  II,  Diastases,  Toxines,  et  Venims,  Paris  (1899)  ;  L.  Michaelis  and 
M.  L.  Menten,  Biochem.  Z.  49,  333  (1913)  ;  J.  M.  Nelson  and  W.  C.  Vosburgh, 
Jour.  Amer.  Chem.  Soc.  39,  790  (1917). 


116  CATALYTIC  ACTION 

amylase,  starch;1  lactase,  lactose;2  maltase,  maltose;2  emulsin, 
glucosides;2  lipase,  esters;3  urease,  urea.4  These  results  can 
be  accounted  for  most  readily  on  the  intermediate  compound 
theory,  each  enzyme  showing  a  maximum  capacity  for  combina- 
tion with  the  substrate,  if  the  latter  is  present  in  large  excess. 
The  rate  of  reaction  will  then  be  proportional  to  the  concentration 
of  the  addition  compound  of  enzyme  and  substrate  if  its  decom- 
position is  slower  than  its  formation  and  a  steady  state  will  then 
exist  with  regard  to  the  combined  condition  of  the  enzyme. 
Either  all  of  the  enzyme,  or  a  constant  fraction  of  it,  will  exist 
all  the  time  combined  with  the  substrate.  This  relation,  obvi- 
ously, will  hold  only  as  long  as  no  interfering  actions  such  as  may 
be  caused  by  the  products  of  the  reaction,  etc.,  occur. 

The  question  of  Schiitz's  rule  may  be  considered  as  an 
example  of  the  modifications  in  the  mathematical  deductions 
made  necessary  at  times  by  complex  experimental  conditions. 
Schiitz  5  found  experimentally  that  in  the  action  of  pepsin  on 
egg  albumin,  the  amounts  of  egg  albumin  digested  (to  peptone) 
in  a  given  time  with  different  quantities  of  pepsin  were  propor- 
tional to  the  square  roots  of  the  concentrations  of  pepsin.  This 
rule  was  found  to  hold  for  the  first  part  (one-third  to  one-half) 
of  the  reaction  by  different  workers.  Also,  the  rule  was  found 
to  hold  for  different  enzymes,  including  lipase  and  diastase,  under 
certain  conditions. 

Arrhenius 6  showed  that  in  the  hydrolysis  of  ethyl  acetate 
present  in  great  excess,  by  ammonia  (or  ammonium  hydroxide), 
the  mathematical  equation  representing  the  change  was  analogous 
to  the  equation  of  Schiitz's  rule  after  the  first  moments  of  the 
reaction.  The  ammonium  ion  of  the  ammonium  acetate  formed 
in  the  reaction  repressed  the  ionization  of  the  ammonium  hy- 
droxide and  therefore  the  concentration  of  the  hydroxyl  ions. 
The  velocity  would  be  inversely  proportional  to  the  amount  of 

1  H.  T.  Brown  and  T.  A.  Glendinning,  J.  Chem.  Soc.  81,  388    (1902)  ;  C.   L. 
Evans,  J.  Physiol.  44,  191   (1912). 

2  E.  F.  Armstrong,  Proc.  Roy.  Soc.  London  (B) ,  73,  500  (1914). 

3  H.  C.  Bradley,  J.  Biol.  Chem.  8,  251   (1910)  ;  G.  Peirce,  Jour.  Amer.  Chem. 
Soc.  32,  1517    (1910)  ;  K.   G.  Falk  and   K.   Sugiura,   Jour.  Amer.   Chem.  Soc.  37, 
217    (1915). 

*D.  D.  van  Slyke  and  G.  E.  Cullen,  J.  Biol.  Chem.  19,  141   (1914). 
5  A   summary    together  with    many   references   is  given   by   H.    Euler,    "Allge- 
meine  Chemie  der  Enzyme,"   1910,   pp.   127-137. 

8gr  A.  Arrhenius,  Medd.  Kong,  vetsakad.  Nobelinst.  (1908)   1. 


ENZYME  ACTIONS  117 

ammonium  acetate  formed  after  the  first  few  minutes  of  the 
reaction.  The  following  general  equation  was  deduced: 

A  loge-r  --  x  =  kqt  (1) 

2\.  -  X 

in  which  A  represents  the  concentration  of  ammonia  at  the  be- 
ginning of  the  reaction,  x  the  quantity  transformed  into  am- 
monium acetate  at  the  time  t,  q,  the  concentration  of  ester,  and  k 
the  reaction  velocity  constant.  Before  x  becomes  too  large  (as 
with  the  enzyme  actions  just  mentioned)  this  equation  reduces 
to  the  form 

x  =  VkAqt  (2) 

which  is  an  expression  of  Schiitz's  rule,  and  which  was  found  to 
hold  for  the  hydrolysis  of  ethyl  acetate  by  ammonia  under  the 
indicated  conditions. 

A  more  exact  equation  apparently  similar  in  form  to  the 
equation  of  Arrhenius  was  deduced  by  Northrop  x  in  connection 
with  the  study  of  the  digestion  of  proteins  by  pepsin  in  order  to 
represent  the  complete  course  of  the  reaction.  The  action  was 
shown  to  be  caused  by  free  pepsin,  and  the  amount  of  free  pepsin, 
after  the  first  few  minutes  was  found  to  be  inversely  proportional 
to  the  amount  of  products.  The  pepsin  was  present  in  solution 
free  or  in  combination  with  the  products  of  hydrolysis  of  the 
protein,  the  relative  concentrations  following  the  law  of  mass 
action.  The  equation  of  Northrop  is  as  follows: 


(3) 


E  representing  the  concentration  of  enzyme.  The  equation  dif- 
fers from  that  of  Arrhenius  in  that  in  the  latter  the  substrate 
concentration  was  assumed  to  remain  constant  while  the  enzyme 
concentration  was  represented  by  the  term  (A  —  x)/x,  and 
in  the  former,  the  substrate  concentration  was  expressed 
by  the  term  q  —  x,  and  the  enzyme  concentration  by  the  term 

1  J.  H.  Northrop,  J.  Gen.  Physiol.  2,  471   (1920). 


118  CATALYTIC  ACTION 

E/x.  Neither  the  Arrhenius  nor  the  Northrop  equation  repre- 
sents the  experimental  facts  for  the  first  few  minutes  of  the 
reaction  (until  the  concentration  of  substrate  decomposed  is 
large,  ten  to  fifteen  times  as  great  as  the  concentration  of  active 
pepsin).  Both  equations  simplify  to  Schiitz's  rule  for  the  next 
thirty  to  forty  per  cent  of  the  reaction,  and  after  that,  when  the 
substrate  is  no  longer  present  in  great  excess  and  its  concentra- 
tion can  no  longer  be  considered  constant  in  the  mathematical 
formulation,  the  Northrop  equation  more  nearly  represents  the 
experimental  facts. 

These  deductions  strictly  speaking  apply  only  to  the  pepsin- 
protein  reaction.  The  general  method  of  treatment  should, 
however,  be  applicable  as  well  to  other  enzyme  actions,  but  only 
after  these  have  been  subjected  to  similar  careful  experimental 
study. 

In  every  discussion  of  chemical  reaction  velocity,  the  ques- 
tion of  the  reaction  in  the  opposite  direction,  or  the  reversibility 
of  the  reaction,  must  be  considered.  It  has  been  found  experi- 
mentally that  a  number  of  enzyme  preparations,  including  lipase, 
emulsin,  trypsin,  pepsin,  kephirlactase,  maltase,  and  oxynitrilase, 
bring  about  syntheses  of  more  complex  bodies  from  the  simpler 
substances  which  were  produced  by  the  actions  of  the  same  or 
similar  preparations.1  Quantitative  work  unfortunately  is  not 
available  relative  to  the  synthetic  actions  of  these  enzyme  prep- 
arations, comparable  to  their  decomposing  actions.  The  pos- 
sible effect  of  the  presence  of  an  enzyme  on  the  equilibrium  con- 
centrations of  the  reacting  constituents  has  also  not  been  sub- 
jected to  careful  study  as  yet. 

The  phenomena  of  enzyme  reactions  which  have  been  de- 
scribed briefly  and  rather  superficially  in  the  last  pages  can  be 
brought  together  so  as  to  indicate,  in  part  at  any  rate,  the  mech- 
anism of  the  changes  and  at  the  same  time  emphasize  again  the 
fact  that  enzyme  actions  belong  to  the  group  of  catalytic  actions, 
and  to  show  as  well  their  relations  to  chemical  reactions  in  gen- 
eral. The  following  scheme  of  equations  represents  some  of  the 
possible  actions  in  a  complex  mixture  such  as  might  occur  in  an 
enzyme  action : 

JFor  references,  cf.  "The  Chemistry  of  Enzyme  Actions,"  pp.  104-5. 


ENZYME  ACTIONS  119 


"Enzyme 

Substrate 
(or  Products) 
Water 


=  Enz.  +  Substr.  +  Water  (a) 

=  Enz.  +  Products  +  Water  (b) 

—  (Enz.  Products)  +  Water  (c) 

=  (Enz.  Substrate)  +  Water  (d) 


(4) 


These  equations  do  not  show  the  actions  of  any  other  substances, 
and,  in  fact,  outline  only  the  simplest  changes  which  can  take 
place  for  enzyme  actions  involving  hydrolysis  reactions.  An 
analogous  set  of  equations  would  be  necessary  to  represent  oxi- 
dation-reduction reactions.  This  set  of  equations  is  similar  in 
form  to  thq  equations  which  were  given  in  a  previous  chapter 
to  represent  the  mechanism  of  chemical  actions. 

Starting  with  the  substances  on  the  right  hand  side  of  equa- 
tion (a),  if  the  products  of  equation  (b)  are  obtained,  the  sim- 
plest case  of  enzyme  action  would  be  represented.  Here,  as  in 
all  examples  of  chemical  kinetics,  if  a  reaction  takes  place  in 
two  (or  more)  stages,  the  reaction  which  has  the  smaller  rate 
is  the  one  to  be  measured  experimentally,  if  the  difference  in 
rates  is  sufficiently  great.  If  this  difference  is  not  large  enough, 
uncertain  and  confusing  results  may  be  obtained  in  the  applica- 
tion of  the  (simplified)  mathematical  formulas  to  the  chemical 
equations  which  may  represent  only  a  part  of  the  chemical 
changes  actually  taking  place.  With  certain  simplified  condi- 
tions, such  as  the  presence  of  a  large  excess  of  substrate,  simpli- 
fied mathematical  expressions  will  be  found  to  hold.  In  such  a 
case,  the  reaction  velocity  measured  is,  as  a  rule,  the  decomposi- 
tion rate  of  the  intermediate  addition  compound,  equation  (b), 
since  the  reaction  represented  by  equation  (a)  has  assumed  a 
steady  state  which  can  be  deduced  from  the  mass  action  relations 
as  indicated  by  Northrop  for  a  special  case.  Another  factor 
which  makes  the  application  of  the  kinetic  equations  uncertain  at 
times  is  the  physical  condition  of  the  enzyme  preparation  and  the 
substrate.  The  enzyme  preparation  is  almost  always  colloidal  in 
character,  while  the  substrate  may  also  be  a  colloid  as  with  pro- 
teins, starches,  and  some  fats.  The  ordinary  application  of  the 
mass  action  expression  may  be  open  to  question  here,  because 
of  the  difficulty  of  determining  the  active  mass  of  the  colloidal 
particle.  The  introduction  of  the  concentration  as  the  active 
mass  does  not  appear  warranted  without  further  direct  evidence. 


120  CATALYTIC  ACTION 

This  general  question  was  discussed  in  an  earlier  chapter  and  will 
be  taken  up  again  under  "Contact  Catalysis"  in  Chapter  VIII. 
At  the  same  time,  it  may  be  mentioned  that  experiments  showed 
a  sucrase  preparation  not  to  be  affected  in  its  activity  whether 
or  not  the  enzyme  was  adsorbed  on  a  solid  like  charcoal,  or  on  a 
colloid  like  saponin,  serum,  or  egg  albumin,  or  distributed  uni- 
formly throughout  the  solution  of  the  substrate ; x  that  with  pep- 
sin, the  state  of  aggregation  of  the  protein,  whether  in  solution 
or  not,  exerted  no  marked  influence  on  the  rate  of  digestion  ;2  and 
that  with  a  castor  bean  lipase  preparation,  whether  the  enzyme 
material  was  dissolved  in  salt  solution  or  suspended  in  the  aque- 
ous solution,  appeared  to  make  little  or  no  difference  in  its 
hydrolyzing  action.3 

Equation  (c)  in  the  general  formulation  represents  the  pos- 
sible interference  due  to  the  enzyme  combination  with  the  prod- 
ucts of  the  decomposition  of  the  substrate.  Obviously  it  is  not 
necessary  for  the  reaction  between  the  products  and  the  enzyme, 
or  the  equilibrium  between  them,  to  pass  through  or  include  the 
intermediate  addition  compound.  An  equation  including  these 
alone  might  be  written,  but  for  the  sake  of  showing  the  general 
relationship,  this  has  not  been  done.  Similarly,  equation  (d) 
shows  the  combination  of  enzyme  and  substrate.  That  such  com- 
binations exist  and  play  an  important  part  at  times  may  be  illus- 
trated by  the  fact  that  a  number  of  enzyme  preparations  have 
been  found  to  be  more  resistant  to  factors  which  cause  inactiva- 
tion  in  the  presence  of  their  substrates  than  in  their  absence. 
This  has  been  found  to  be  true,  for  example,  for  sucrase,4  tryp- 
sin,5  and  amylase.6 

The  significance  of  the  enzyme  term  in  the  equations  re- 
quires some  explanation.  The  exact  concentration  to  be  used  in 
the  kinetic  and  other  formulations  must  be  determined  for  each 
individual  action.  The  hydrogen  ion  concentration  of  the  me" 

1  J.  M.  Nelson  and  E.  G.  Griffin,  Jour.  Amer.  Chem.  Soc.  38,  1109    (1916)  ; 
cf.   also   J.   M.   Nelson   and   D.   I.    Hitchcock,  Jour.  Amer.    Ghem.    Soc.   Jj3,   1956 
(1921). 

2  J.  H.  Northrop,  J.~  Gen.  Physiol.  1,  607   (1919)  ;  W.  E.  Ringer,  Z.  physiol. 
Ghem.  95,  195   (1915). 

*K.  G.  Falk,  Jour.  Amer.  Chem.  Soc.  37,  226   (1915). 

4  C.  O'Sullivan  and  F.  W.  Tompson,  J.  Ghem.  Soc.  57,  834   (1890). 

5W.  M.  Bayliss  and  E.  H.  Starling,  J.  Physiol.  30,  61  (1903);  cf.  also 
W.  M.  Bayliss,  Proc.  Roy.  Soc.  London  (B)  84,  81  (1911). 

6  T.  B.  Osborne  and  G.  F.  Campbell,  Jour.  Amer.  Chem.  Soc.  18,  536  (1896)  ; 
H.  C.  Sherman  and  co-workers,  series  of  papers  in  Jour.  Amer.  Ghem.  Soc. 
1914-1921. 


ENZYME  ACTIONS  121 

dium  is  naturally  the  first  factor  to  be  thought  of  in  this  con- 
nection, because  of  the  optimum  conditions  which  have  been 
found  to  exist  for  all  enzyme  actions.  Similarly  there  may  be 
optimum  conditions  for  the  substrate,  as  with  the  protein  ions 
which  Northrop  showed  were  essential  for  optimum  pepsin  ac- 
tion. Also,  there  may  be  optimum  conditions  for  the  decomposi- 
tion of  the  intermediate  addition  compound  which  may  be  differ- 
ent from  the  optimum  conditions  for  its  formation. 

The  actions  of  foreign  substances,  foreign  in  the  sense  that 
they  are  not  involved  in  the  simplest  formulations  of  the  reac- 
tions, may  exert  considerable  influence.  Thus,  they  may  com- 
bine with  the  enzyme,  a  reaction  similar  to  equation  (c)  of  the 
general  formulation,  in  a  reversible  or  irreversible  way,  and  re- 
move it  from  the  sphere  of  action;  they  may  combine  with  the 
products  of  the  reaction,  and  by  removing  them  favor  the  en- 
zymic  decomposition;  they  may  combine  with  the  substrate,  or 
cause  a  change  in  it  which  would  interfere  with  the  reaction; 
etc.  In  every  case,  however,  the  interpretation  of  the  reaction 
is  a  chemical  one.  The  changes  are  analogous  to  changes  in  ordi- 
nary, well-known  chemical  reactions.  The  exact  chemical  formu- 
las and  structures  cannot  be  given  here  and  recourse  must  be 
had,  temporarily  it  is  to  be  hoped,  to  more  or  less  definite  sym- 
bols. With  these  limitations  in  mind,  the  application  of  the 
ordinary  chemical  laws  and  theories  may  be  carried  out  in  ex- 
actly the  same  way  as  with  chemical  reactions  involving  sub- 
stances of  simpler  compositions  and  structures. 

In  this  chapter  a  brief  review  of  enzyme  actions  considered 
as  a  group  of  catalytic  actions  was  presented.  Many  of  the 
chemical  changes  included  in  enzyme  actions  may  also  be  brought 
about  by  hydrogen  and  hydroxyl  ions,  or  perhaps  better,  may 
involve  these  ions.  The  actions  of  hydrogen  and  hydroxyl  ions 
were  therefore  included  in  the  discussions  in  this  chapter,  but 
not  with  the  intention  of  belittling  their  importance  and  signifi- 
cance by  apparently  considering  them  incidentally.  Their  actions 
have  been  treated  in  detail  in  various  publications  by  a  number 
of  workers.  Unfortunately,  definite  conclusions  have  not  been 
reached  in  comparatively  simple  reactions  such  as  the  hydrolysis 
of  sucrose  or  of  esters  by  these  ions.  In  fact,  the  determination 
of  the  degree  of  ionization  has  not  been  satisfactorily  elucidated 


122  CATALYTIC  ACTION 

so  as  to  make  possible  the  development  of  a  general  viewpoint. 
On  the  other  hand,  the  work  of  Stieglitz  on  the  decomposition 
of  imido  esters  and  related  compounds  does  give  a  definite  view- 
point from  which  to  consider  the  so-called  "catalytic"  actions 
of  hydrogen  ions  and  brings  these  actions  into  line  with  chemical 
reactions  in  general.  His  general  theoretical  conceptions  may  be 
carried  over  to  reactions  such  as  the  hydrolysis  of  esters.  Chemi- 
cal evidence  is  at  hand  to  bear  out  a  number  of  the  deductions 
but  the  exact  formulation  of  the  changes  to  be  included  in  such 
reactions  as  the  hydrolysis  of  esters  is  not  certain  even  under 
these  conditions.  It  was  considered  advisable  therefore  to  omit 
the  detailed  discussion  of  these  reactions  in  the  present  connec- 
tion. Enzyme  actions  being  of  more  complex  nature  offer  more 
points  of  contact  with  other  branches  of  chemistry  and  related 
sciences  and  indicate  possibilities  of  development,  some  of  which 
will  be  considered  in  the  following  chapter.  The  present  chapter 
was  therefore  headed  "Enzyme  Actions"  and  hydrogen  and 
hydroxyl  ion  actions  considered  briefly  in  the  discussion  of  the 
chemical  changes  involved.  At  some  other  time  and  in  some 
other  connection  it  is  possible  that  the  interests  of  the  writer 
will  be  shifted  so  that  a  chapter  such  as  this  would  be  headed 
"Hydrogen  and  Hydroxyl  Ion  Actions"  and  enzyme  actions  con- 
sidered in  connection  with  these.  The  actions  of  other  ions  or 
molecules  might  also  be  considered  as  the  dominant  actions.  In 
any  given  case,  the  personal  preference,  at  the  time,  of  the  one 
presenting  the  subject  will  be  the  decisive  factor  in  the  method 
of  treatment  to  be  followed. 


Chapter  VII. 
A  Chemical  Interpretation  of  Life  Processes. 

In  the  consideration  of  the  chemical  changes  which  take  place 
in  nature,  a  classification  which  has  been  suggested  in  the  past 
and  for  which  there  is  considerable  justification,  divides  reactions 
into  two  groups:  those  occurring  in  living  matter,  and  those  oc- 
curring in  non-living  matter.  It  is  evident  that  many  of  the 
chemical  reactions  which  occur  in  living  matter  may  be  caused 
to  take  place  by  artificial  means  in  non-living  matter.  At  the 
same  time,  the  changes  in  living  matter,  during  the  time  that 
such  matter,  by  common  consent,  is  said  to  be  living,  possess 
characteristics  which,  when  considered  from  certain  points  of 
view,  appear  to  set  off  such  changes  from  those  occurring  in  non- 
living matter.  In  this  chapter,  an  attempt  will  be  made  first  to 
present  the  chemical  phenomena  of  living  matter  from  a  strictly 
chemical  point  of  view,  to  show  the  analogies  between  the  two 
groups  of  reactions,  interpreting  both  on  the  basis  of  the  same 
fundamental  laws  of  chemistry,  and  finally  to  present  from  this 
chemical  viewpoint  certain  developments  of  the  phenomena  of 
life  processes  in  connection  with  related  branches  of  science. 

In  order  to  develop  these  views  in  a  systematic  way,  it  will 
be  necessary  to  refer  to  and  to  repeat  some  of  the  classifications 
and  theories  which  were  presented  in  the  earlier  chapters  of  this 
book  as  well  as  in  other  connections.  The  topics  under  discus- 
sion involve  chemical  change.  The  development  of  a  general 
theory  of  chemical  reactions  which  would  include  reactions  both 
of  inorganic  chemistry  as  well  as  of  organic  chemistry  is  a  neces- 
sary forerunner  of  the  further  views.  A  theory  of  this  sort  is 
the  "Addition  Theory  of  Chemical  Reactions"  which  was  pre- 
sented in  a  somewhat  extended  form  elsewhere.1  According  to 
this  theory,  an  addition  compound  is  the  first  product  formed 

1  "Chemical  Reactions  ;  Their  Theory  and  Mechanism." 

123 


124  CATALYTIC  ACTION 

when  two  or  more  molecules  react.  This  addition  compound  may 
then  decompose  or  react  farther  in  a  number  of  different  ways, 
the  conditions,  such  as  the  relative  concentrations  of  the  reacting 
constituents,  removal  of  products,  etc.,  determining  in  any  one 
case  the  actual  products  obtained.  If  a  reaction  does  not  attain 
equilibrium  and  two  or  more  decomposition  reactions  are  pos- 
sible for  the  addition  compound,  other  things  being  equal,  the 
reaction  taking  place  with  the  greatest  velocity  will  be  the  one 
observed  experimentally.  With  a  monomolecular  reaction,  the 
velocity  is  directly  dependent  in  some  way  on  the  internal  elec- 
tronic structure  of  the  reacting  molecule.  Some  of  the  more 
recent  theories  which  have  attempted  explanations  of  the  phe- 
nomena of  monomolecular  reactions  were  presented  in  Chapter 
IV.  At  the  same  time,  the  decomposition  of  the  intermediate 
addition  compound  in  the  reactions  of  higher  orders  is  funda- 
mentally in  every  case  a  monomolecular  reaction  and  dependent 
undoubtedly  upon  the  electronic  configuration  within  the  mole- 
cule. Reaction  in  any  case  involves  rearrangement  of  the  elec- 
trons, their  distributions  and  motions,  accompanied  by  separa- 
tion of  atoms  and  simpler  molecules.  With  more  complex  addi- 
tion compounds,  greater  numbers  of  modes  of  decomposition 
would  be  possible.  A  number  of  such  reactions  illustrating  the 
possibilities  of  the  addition  theory  have  been  presented  in  dif- 
ferent connections. 

The  second  step  in  the  development  includes  the  introduc- 
tion of  catalysts  into  the  treatment  of  chemical  reactions.  From 
the  considerations  presented  in  the  earlier  chapters  it  will  have 
been  made  evident  that  no  generally  satisfactory  classification  or 
theory  of  catalytic  actions  as  distinct  from  other  reactions  ex- 
ists, but  that  whatever  definition  or  description  is  used,  the  ordi- 
nary laws  and  relations  of  chemistry  are  applicable.  (It  is  well 
to  repeat  this  fact  frequently  because  of  the  confusion  created 
by  some  authors  in  writing  of  catalytic  reactions.)  The  phe- 
nomena of  catalysis  may  make  themselves  evident  experimen- 
tally according  to  any  of  the  definitions  or  classifications  pro- 
posed, by  a  change  in  the  velocity  of  a  given  chemical  reaction 
upon  the  addition  of  the  catalyst,  or  where  two  or  more  reactions 
are  possible,  by  one  being  favored  over  the  other  in  the  sense 
that  the  addition  of  the  catalyst  results  in  the  products  of  one 


CHEMICAL  INTERPRETATION  OF  LIFE  PROCESSES  125 

of  the  possible  reactions  being  increased  more  than  the  products 
of  the  other  reactions.  For  the  sake  of  completeness,  it  may  be 
mentioned  that  a  catalyst  may  be  involved  in  a  reaction  with- 
out causing  these  effects  according  to  certain  views,  and  also 
that  radiations  of  various  frequencies  might  be  included  under 
the  term  catalysts.  The  theory  of  catalytic  actions  which  ap- 
pears to  the  writer  to  cover  the  facts  most  satisfactorily  is  that 
which  considers  that  in  a  catalytic  reaction  the  chemical  com- 
position of  one  of  the  initial  and  final  products  is  the  same. 
The  reaction  may  take  place  in  two  or  more  steps.  The  change 
in  velocity  is  only  one  of  the  signs  to  show  that  a  reaction  is 
catalytic.  A  change  in  the  proportions  of  reaction  products  is 
another.  These  relations  have  been  developed  in  several  places 
in  this  book  as  well  as  elsewhere. 

The  next  step  involves  the  consideration  of  enzymes  as  cata- 
lysts. Enzymes  are  catalysts  produced  by  living  matter,  which, 
even  in  the  absence  of  the  life  process,  are  able  to  exert  catalytic 
actions  on  more  or  less  definite  chemical  reactions.  The  veloci- 
ties of  these  reactions  are  changed,  or  the  products  may  be  ob- 
tained in  different  proportions  in  the  absence  and  in  the  pres- 
ence of  the  enzyme  preparations.  These  enzymes  appear  to  pos- 
sess certain  characteristics  which  are  generally  associated  with 
living  matter.  They  are  inactivated  under  conditions  which  de- 
stroy life,  such  as  heat,  addition  of  certain  substances  which 
may  be  said  to  act  as  poisons,  etc.  Such  behavior  is  not,  how- 
ever, limited  to  enzymes  or  catalysts  produced  by  living  matter. 
The  striking  investigations  of  G.  Bredig x  and  his  co-workers  on 
"Inorganic  Ferments"  must  be  spoken  of  in  this  connection. 
Colloidal  solutions  of  metals  such  as  platinum,  gold,  silver,  etc., 
prepared  by  passing  an  electric  arc  between  two  electrodes  of 
the  metal  under  water,  increase  the  rate  of  decomposition  of 
hydrogen  peroxide  solutions.  The  enzyme  catalase  exerts  a 
similar  action.  The  activities  of  both  the  inorganic  ferment  and 
the  enzyme  were  increased  by  increase  in  temperature  up  to  a 
certain  optimum  and  then  were  decreased,  each  ferment  having 
its  own  optimum  temperature.  Small  quantities  of  certain  sub- 

»G.  Bredig  and  R.  Muller  von  Berneck,  Z.  phyaik.  Chem.  31,  258  (1899)  ; 
G.  Bredig  and  K.  Ikeda,  Z.  physik.  Chem.  37.  1  (1901)  ;  G.  Bredig  and  W.  Rein- 
ders  Z  ptiysik.  ('hem.  37,  323  (1901)  ;  G.  Bredig,  Ergebnisse  der  Physiologic  I, 
134  '(1902)  ;  GT  Bredig  and  M.  Fortner,  Ber,  37,  798  (1904), 


126  CATALYTIC  ACTION 

stances  such  as  hydrogen  cyanide,  hydrogen  sulfide,  mercuric 
chloride,  etc.,  inhibited  the  actions  of  the  inorganic  ferments  and 
were  therefore  called  poisons.  Other  substances  such  as  formic 
acid,  hydrazine,  etc.,  increased  the  activities.  The  analogies 
between  the  actions  of  the  inorganic  ferments  and  some  of  the 
actions  of  certain  enzymes  are  striking,  but  must  not  be  pushed 
too  far  at  present.  While  the  chemical  compositions  and  struc- 
tures of  most  of  the  enzymes  are  practically  unknown,  the  com- 
positions of  metallic  colloidal  solutions  are  not  known  to  any 
satisfactory  extent  either,  since  small  amounts  of  "foreign"  sub- 
stances may  exert  considerable  influence  on  their  stability  and 
behavior  aside  from  their  catalytic  actions.1 

Before  proceeding  to  the  next  step  in  the  developments,  a  few 
words  may  be  said  relative  to  "hormones"  and  so-called  "vita- 
mins" or  perhaps  better  "food  hormones."  The  study  of  the 
actions  included  under  these  terms  has  been  carried  on  exten- 
sively in  the  past  ten  years,  and  the  literature,  especially  of  the 
latter  subject,  is  almost  overwhelming.  These  actions  have  fre- 
quently been  compared  to  enzyme  actions.  Just  how  far  the 
comparison  is  justified  cannot  profitably  be  entered  into  in  the 
present  connection.  Enzyme  actions  can  be  studied  with  definite 
chemical  reactions  outside  the  living  organism  and  can  conse- 
quently be  made  to  yield  comparable  quantitative  data  upon 
which  to  base  conclusions.  Such  chemical  reactions  have  not  as 
yet  been  found  for  studying  hormones  and  vitamins,  and  all 
the  results  available  at  present  for  these  have  been  obtained 
by  means  of  studies  in  vivo.  All  of  these  actions,  including 
enzyme,  hormone  and  vitamin,  may  be  considered  to  be  cata- 
lytic. All  of  them  retain  certain  properties  which  are  derived 
directly  from  the  life  processes  of  which  they  form,  if  not  the 
most  important  feature  chemically,  at  least  an  important  part. 
These  properties  include  the  temperature  conditions  for  optimum 
action,  conditions  for  inactivation,  etc.  These  similarities  are 
not,  however,  sufficient  reason  for  placing  the  actions  of  hor- 
mones and  vitamins  in  the  same  class  as  enzymes  until  more  is 
known  of  their  behavior.  In  view  of  the  lack  of  definite  chemi- 
cal knowledge  concerning  them  and  their  actions,  they  will  not 

JCf.  H.  T.  Beans  and  Hf  E.  Eastlack,  Jour.  Amer.  CJicm.  Soc.  37,  26G7 
(J915), 


CHEMICAL  INTERPRETATION  OF  LIFE  PROCESSES  127 

be  considered  farther  here.  Attention  is  directed  to  them  since 
they  form  a  group  of  catalysts  in  living  matter  which  parallel 
the  enzymes,  and  which  may  ultimately  be  classed  with  them, 
both  as  separate  groups  of  a  larger  division  comprising  reactions 
taking  place  in  life  processes. 

To  return  to  enzyme  actions  in  life  processes,  the  chemical 
equations  (4)  which  were  given  in  the  last  chapter  as  repre- 
senting some  of  the  possible  changes  may  be  repeated  as  follows: 

=  Enz.  +  Substr.  +  Water  (a) 


"Enzyme 
Substrate 
(or  Products) 
Water 


=  Enz.  +  Products  +  Water  (b) 

=  (Enz.  Products)  +  Water  (c) 

=  (Enz.  Substr.)  +  Water  (d) 


(1) 


It  will  be  recalled  that  with  a  complex  protein,  for  example,  a 
number  of  equations  (symbolized  by  equation  (b))  represent- 
ing different  sets  of  products  would  be  possible,  the  reactions 
observed  or  predominating  depending  upon  the  special  enzyme 
and  substrate  preparations,  and  upon  the  relative  velocities  of 
the  reactions  and  the  conditions  under  which  these  are  pro- 
ceeding. 

The  most  striking  feature  of  the  chemical  changes  occurring 
in  living  matter,  and  in  fact  of  life  processes  themselves,  is  their 
continuity.  As  long  as  life  continues  definite  chemical  changes 
occur.  The  same  or  similar  substances  are  transformed  chemi- 
cally to  produce  the  definite  chemical  substances  which  are  essen- 
tial to  the  life  process  of  the  given  species.  As  outlined  in  the 
preceding  chapter,  the  presence  of  specific  enzymes  causes  the 
reactions  to  follow  the  required  course  and  to  yield  the  products 
essential  for  the  life  process.  In  order  to  have  the  life  process 
continuous,  the  formation  of  enzymes  in  the  chemical  reactions 
is  necessary.  That  is  to  say,  an  enzyme  causes  certain  reaction 
products  to  predominate  in  a  complex  chemical  reaction.  Under 
the  conditions  of  most  of  such  reactions,  certain  other  products 
will  be  eliminated  or  discarded,  at  the  same  time  that  the  original 
enzyme  substance  may  be  inactivated  or  destroyed.  Fresh  en- 
zyme must  therefore  make  its  appearance,  and  this  must  neces- 
sarily arise  from  the  products  of  the  reactions  which  had  pre- 
viously taken  place.  This  would  signify  according  to  equations 
(1)  that  the  enzyme  involved  in  the  reaction  would  be  inacti- 


128  CATALYTIC  ACTION 

vated  or  destroyed  by  further  chemical  action,  but  that  the  sub- 
stances symbolyzed  by  "Products"  would  either  contain  newly 
formed  enzyme  or  contain  material  which  on  further  reaction 
would  be  transformed  into  enzyme.  These  enzyme  reactions 
may  also  proceed  in  a  way  to  produce  complex  bodies  from  simple 
substances,  the  substrate  being  the  latter,  the  products  the  for- 
mer. Enzymes  show  these  synthesizing  actions  as  well  as  the 
decomposing  actions.  Because  of  the  experimental  conditions 
generally  employed  in  studying  enzyme  actions  in  vitro,  the  ex- 
perimental results  obtained  refer  mainly  to  the  decomposing 
actions,  but  it  is  evident  that  in  the  living  animal  or  plant  the 
synthetic  actions  of  enzymes  are  of  equal  importance. 

The  formation  of  fresh  enzyme  material  as  part  of  the  prod- 
ucts of  an  enzyme  action  furnishes  a  special  case  of  catalytic 
acceleration  described  in  Chapter  II.  If  old  enzyme  is  inacti- 
vated or  destroyed  as  rapidly  as  fresh  enzyme  is  produced,  a 
steady  state  is  reached.  If  fresh  enzyme  is  produced  more  rap- 
idly than  old  enzyme  is  destroyed,  a  state  of  increased  change  or 
growth,  either  normal  or  pathologic,  may  result. 

Attention  is  focussed  here  on  the  actions  of  the  enzymes  in 
living  matter.  The  higher  forms  of  living  matter  are  of  great 
complexity  when  considered  from  such  a  chemical  point  of  view. 
The  existence  of  this  complexity  must  be  recognized  but  at  the 
same  time  will  not  be  entered  into  directly  in  connection  with 
the  interpretation  of  the  changes  on  the  basis  of  the  enzyme 
actions.  Thus  it  will  not  be  practicable  to  enter  into  such  ques- 
tions as  the  influence  which  may  be  exerted  by  the  form  of  the 
living  matter  involving  such  relations  as  the  possibility  of  form- 
ing an  enzyme  in  one  part  of  the  organism  and  exerting  its  action 
in  another  part,  etc.  The  discussion  will  be  limited  to  the  possi- 
bility of  a  chemical  interpretation  of  the  relations  and  of  the 
chemical  mechanism  which  may  be  involved. 

Outside  directive  agencies  of  chemical  character  apparently 
are  not  required  to  produce  the  special  enzymes  needed  to  bring 
about  the  chemical  reactions  necessary  for  the  continuation  of 
life  processes.  Life  processes  in  chemical  terms  may  therefore 
be  said  to  consist  of  self-perpetuating  cycles  of  definite  chemical 
reactions  producing,  among  other  products,  enzymes  which  favor 
the  definite  chemical  reactions,  etc.  Any  one  cycle  will  continue 


CHEMICAL  INTERPRETATION  OF  LIFE  PROCESSES  129 

as  long  as  the  suitable  substances  (substrates)  are  provided  and 
the  external  conditions  remain  unchanged  or  favorable.  If  a 
change  occurs  in  the  sense  that  a  different  substrate  is  added,  the 
products  not  removed  as  before,  foreign  substances  which  them- 
selves are  not  changed  by  the  enzyme  action  added,  or  external 
conditions  such  as  temperature,  etc.,  changed,  then  the  enzyme 
which  exerted  its  action  before  may  be  entirely  inactive  or  still 
be  active  but  may  cause  different  products  to  be  formed.  In  the 
first  case  the  life  process  ceases.  In  the  second  case,  with  differ- 
ent products  the  fresh  enzyme  produced  may  also  be  different  and 
then  itself  produce  different  changes  in  the  substrate  upon  which 
it,  in  turn,  acts.  The  nature  of  the  substrate  provided,  the  ex- 
ternal conditions  which  may  be  grouped  under  the  term  environ- 
ment, and  the  enzyme  acting  and  produced,  will  determine  the 
actual  life  process  occurring.  With  small  and  temporary  varia- 
tions in  substrate  and  external  conditions,  the  enzyme  actions 
and  enzyme  produced  would  not  be  expected  to  depart  much 
from  the  original  enzyme  and  its  actions,  and  will  return  to  these 
when  the  former  conditions  are  restored.  With  small  changes  in 
substrate  or  in  external  conditions,  a  change  in  enzyme  and 
enzyme  actions  may  occur,  and  if  the  former  changes  are  con- 
tinuous within  limits,  the  latter  actions  would  be  modified  con- 
tinuously but  not  destroyed.  This  behavior  may  perhaps  be  a 
chemical  interpretation  of  growth  in  living  matter.  Here  again 
the  great  complexity  of  many  forms  of  living  matter  may  make 
it  difficult  to  accept  such  a  comparatively  simple  viewpoint  or 
explanation  for  the  many  interrelated  and  mutually  dependent 
chemical  changes  which  are  taking  place.  It  can  only  be  said 
that  if  a  chemical  interpretation  is  attempted,  and  fundamen- 
tally the  changes  involved  are  chemical  in  character,  a  start  must 
be  made  somewhere,  and  enzyme  actions  at  the  present  time 
offer  the  most  promise.  Simple  explanations  of  apparently  com- 
plex phenomena  may  not  be  correct,  but  at  any  rate  they  give 
a  working  basis,  and  may  be  used  until  better  explanations  are 
available. 

The  changes  in  substrate,  environment,  and  enzyme  character 
which  have  been  spoken  of  may  be  considered  somewhat  farther 
since  they  lead  to  interesting  conclusions.  The  orderly  continu- 


130  CATALYTIC  ACTION 

ous  change  which  is  known  as  growth  represents  one  set  of  pos- 
sible phenomena.     Similar  changes  not  accompanied  by  growth 
may  also  occur.    The  changes  in  substrate  and  in  enzyme  are 
reciprocal — they  mutually  influence  each  other  when  they  occur. 
The  changes  in  enzymes  and  in  enzyme  actions  cannot  be  con- 
trolled directly,  but  the  changes  in  substrate  and  in  environment 
permit  of  a  certain  amount  of  control.    This  will  be  illustrated 
presently  in  considering  some  of  the  experimental  evidence  re- 
lating to  these  phenomena.     If  the  changes  in  substrate  or  in 
environment  or  in  both  are  small  and  continuous,  and  change 
in  enzyme  and  enzyme  action  occurs,  then  the  phenomenon  would 
be  considered  as  adaptation  to  the  environment.    The  continuous 
modification  of  substrate  or  of  environment  or  of  both  would  then 
result  in  a  different,  or  a  modified,  chemical  reaction,  this  in  turn 
in  a  different  enzyme  and  enzyme  action,  the  modifications  in- 
creasing progressively  until  the  new  cycle  conforms  to  the  new 
conditions  imposed  from  without.     If  several  different  cycles  of 
changes  could  occur  under  the  new  conditions,  then  it  might  be 
expected  that  they  would  show  different  degrees  of  adaptability, 
and  only  certain  of  the  cycles  adapt  themselves.     This  might  be 
considered  to  be  the  chemical  significance  of  the  term  "survival 
of  the  fittest."    Further,  these  changes  when  occurring  in  the 
complex  structures  of  highly  organized  living  matter  of  plants 
and  animals  may  result  in  modifying  their  forms  and  reactions.1 
Such  changes  might  be  considered  as  developments  of  the  forms 
of  living  matter.     Fundamentally,  they  are  changes,  but  they 
may  also  be  called  developments.    In  the  past  fifty  years  the  word 
evolution  has  supplanted  the  vaguer  term  development,  so  that 
the  views  here  outlined  might  be  considered  to  represent  a  chem- 
ical interpretation  of  the  mechanism  of  evolution. 

In  this  connection  attention  may  be  called  to  the  fact  that 
while  the  cell  is  the  biological  unit  of  living  matter,  enzymes  may 
be  considered  chemically  to  form  the  distinctive  characteristics 
of  living  matter  since  they  are  the  directive  agencies  controlling 
the  course  of  the  chemical  changes.  The  composition  of  living 
matter  is  fixed  by  the  combinations  of  atoms  of  elements  in  cer- 
tain ways  to  form  molecules;  the  characteristic  structures  of 

*Cf.  E.  S.  Goodrich,  Science  5%,  529  (1921)  ;  "S.owe  Problems  in  Evolution." 


CHEMICAL  INTERPRETATION  OF  LIFE  PROCESSES  131 

living  matter  are  determined  by  cells  and  micelles.1  From  the 
chemical  point  of  view  as  outlined,  enzymes  are  the  controlling 
factors  of  life  processes,  and  distinguish  the  chemical  reactions 
of  such  processes  from  those  of  inanimate  nature  because  of  the 
self-perpetuating  cycles  of  changes  which  they  control. 

In  speaking  of  enzymes  and  their  actions  in  this  way,  the 
conditions  under  which  enzymes  can  act  must  be  kept  in  mind. 
Cells  have  been  termed  the  biological  units  of  living  matter,  but 
except  under  suitable  conditions,  cells  do  not  show  the  distin- 
guishing feature  of  living  matter  in  contrast  to  non-living  matter, 
that  is,  the  ability  to  reproduce  or  the  continuity  of  the  changes. 
Similarly,  enzymes  manifest  their  actions  as  directive  agents 
under  suitable  conditions.  Certain  of  the  properties  and  actions 
of  enzyme  preparations  may  be  studied  by  means  of  simple  lab- 
oratory experiments,  but  the  conditions  under  which  they  might 
be  expected  to  show  the  phenomena  associated  with  life  processes 
have  not  as  yet  been  realized  experimentally  apart  from  the  liv- 
ing organism.  These  conditions  include  such  factors  as  supply 
of  proper  substrates  in  the  necessary  concentrations,  removal  of 
part  of  the  products  of  the  reactions,  absence  of  interfering  sub- 
stances, etc.  Some  of  these  factors  will  be  considered  somewhat 
farther  in  the  following  chapter  on  "Contact  Catalysis"  in  con- 
nection with  the  part  played  by  membranes  with  reactions  in- 
volving substances  present  in  several  phases.  This  side  of  the 
problem  will  be  ignored  in  the  present  connection,  but  with  no 
intention  of  minimizing  its  importance. 

The  study  of  life  processes  from  a  chemical  point  of  view 
lias  been  pursued  by  J.  Loeb  for  a  considerable  number  of  years. 
The  following  quotations  from  some  of  his  writings  are  of  interest 
especially  in  connection  with  the  problems  considered  in  the  pre- 
ceding pages. 

"We  know  that  growth  and  development  in  animals  and  plants  are 
determined  by  definite  although  complicated  series  of  catenary  chemical 
reactions,  which  result  in  the  synthesis  of  a  definite  compound  or  group 
of  compounds,  namely,  nucleins. 

"The  nucleins  have  the  peculiarity  of  acting  as  ferments  or  enzymes  for 
their  own  synthesis.  Thus  a  given  type  of  nucleus  will  continue  to  syn- 
thesize other  nuclein  of  its  own  kind.  This  determines  the  continuity  of  a 

1  The  word  "micelle"'  may  be  taken  to  signify  the  unit  of  colloidal  protein 
or  other  material  in  a  complex  molecule  or  particle.  It  represents  a  more  or 
less  definite  unit  of  matter,  not  as  rigidly  definable  at  present  as  the  molecule, 
but  useful  in  considering  colloids  and  their  behaviors. 


132  CATALYTIC  ACTION 

species;  since  each  species  has,  probably,  its  own  specific  nuclein  or  nuclear 
material.  But  it  also  shows  us  that  whoever  claims  to  have  succeeded  in 
making  living  matter  from  inanimate  will  have  to  prove  that  he  has  suc- 
ceeded in  producing  nuclear  material  which  acts  as  a  ferment  for  its  own 
synthesis  and  thus  reproduces  itself.  Nobody  has  thus  far  succeeded  in 
this,  although  nothing  warrants  us  in  taking  it  for  granted  that  this  task  is 
beyond  the  power  of  science.1" 

"The  essential  difference  between  living  and  non-living  matter  consists 
then  in  this :  the  living  cell  synthetizes  its  own  complicated  specific  material 
from  indifferent  or  non-specific  simple  compounds  of  the  surrounding'  me- 
dium, while  the  crystal  simply  adds  the  molecules  found  in  its  supersatu- 
rated solution.  This  synthetic  power  of  transforming  small  "building 
stones"  into  the  complicated  compounds  specific  for  each  organism  is  the 
"secret  of  life"  or  rather  one  of  the  secrets  of  life. 

"What  clew  have  we  in  regard  to  the  nature  of  this  synthetic  power? 
We  know  that  the  comparatively  great  velocity  of  chemical  reactions  in  a 
living  organism  is  due  to  the  presence  of  enzymes  (ferments)  or  to  catalytic 
agencies  in  general.  Some  of  these  catalytic  agencies  are  specific  in  the 
sense  that  a  given  catalyzer  can  accelerate  the  reaction  of  only  one  step  in 
a  complicated  chemical  reaction.  While  these  enzymes  are  formed  by  the 
action  of  the  body,  they  can  be  separated  from  the  body  without  losing 
their  catalytic  efficiency.2 

"The  constant  synthesis  then  of  specific  material  from  simple  compounds 
of  a  non-specific  character  is  the  chief  feature  by  which  living  matter 
differs  from  non-living  matter."3 

Some  of  the  quantitative  data  published  by  Loeb  in  recent 
years  bearing  upon  the  changes  occurring  in  life  processes  will 
be  presented.  The  study  4  of  the  effect  of  temperature  on  the 
duration  of  life  of  the  fruit  fly  (Drosophila)  freed  from  all  micro- 
organisms and  with  an  adequate  food  supply  gave  a  temperature 
coefficient  of  between  two  and  three  for  a  10°  rise  in  tempera- 
ture, a  value  of  the  same  order  of  magnitude  as  the  temperature 
coefficient  of  a  chemical  reaction.  The  ratio  of  the  duration  of 
life  of  the  insect  to  the  duration  of  the  larval  stage,  and  the 
ratio  of  the  larval  to  the  pupa  stage,  were  found  to  be  approxi- 
mately constant  for  all  temperatures  studied.  It  was  suggested 
that  the  duration  of  life  was  determined  by  the  production  of 
a  substance  leading  to  old  age  and  natural  death  or  by  the  de- 
struction of  a  substance  or  substances  which  normally  prevent 
old  age  and  natural  death. 

The    quantitative    study    of    regeneration    in    the    stem    of 

1 J.  Loeb,  "The  Mechanistic  Conception  of  Life."  University  of  Chicago 
Press,  Chicago,  1912.  P.  227. 

2  J.  Loeb,  "The  Organism  as  a  Whole  From  a  Physicochemical  Viewpoint," 
G.  P.  Putnam's  Sons,  New  York  and  London,  1916.  P.  23. 

8  J.  Loeb,  "The  Organism  as  a  Whole  From  a  Physicochemical  Viewpoint," 
P.  29. 

*J.  Loeb  and  J.  H.  Northrop,  Proc.  Nat.  Acad.  Sci.  3,  382  (191T)  ;  J.  Biol, 
Cliem.  32,  103  (1917). 


CHEMICAL  INTERPRETATION  OF  LIFE  PROCESSES  133 

Bryophyllum  calycinum  gave  the  following  results.1  Equal 
masses  of  sister  leaves  produced  equal  masses  of  shoots  and  roots 
in  equal  times  and  under  the  same  conditions,  regardless  of  the 
number  of  shoots  produced.  The  mass  of  shoots  and  roots  pro- 
duced by  different  masses  of  sister  leaves  in  equal  times  and 
under  equal  conditions  was  approximately  in  direct  proportion  to 
the  masses  of  the  leaves.  When  a  piece  of  stem  inhibited  the 
production  of  shoots  and  roots  in  a  leaf  of  Bryophyllum  connected 
with  it,  the  stem  gained  in  mass  an  amount  approximately  equal 
to  the  mass  of  shoots  and  roots  the  leaf  would  have  produced  if 
it  had  been  detached  from  the  stem.  The  inhibitory  influence  of 
the  stem  upon  the  formation  of  roots  and  shoots  in  the  leaf  ap- 
parently was  due  to  the  fact  that  the  material  available  and  re- 
quired for  this  process  flowed  into  the  stem.  In  other  words,  the 
principle  of  chemical  mass  action  was  found  to  apply  to  the 
problem  of  growth  in  the  given  case. 

The  phenomena  which  have  been  included  under  the  terms 
"Forced  Movements"  and  "Tropisms"  whose  scientific  impor- 
tance and  significance  were  developed  mainly  by  J.  Loeb  may 
also  be  spoken  of  in  this  connection.2  The  tropisms  which  have 
been  studied  include  galvanotropism,  heliotropism,  geotropism, 
stereotropism,  chemotropism,  and  thermotropism.  Explanations 
for  the  actions  observed  under  the  influence  of  the  various  forms 
of  energy  were  based  upon  the  chemical  properties  of  the  sub- 
stances affected  and  possible  changes  in  the  active  masses 
of  the  substances  under  the  different  conditions,  and  satisfactory 
conclusions  reached.  It  is  possible  that  enzyme  actions  also  are 
involved,  perhaps  after  or  at  the  same  time  as  the  direct  chemical 
effects  which  were  observed,  and  that  the  reciprocal  actions  of 
the  changes  in  substances  and  the  enzyme  actions  are  responsible 
for  the  changes  observed. 

In  all  of  the  work  described,  the  changes  are  shown  to  follow 
the  chemical  mass  action  principle.  The  question  still  remains 
open,  however,  whether  it  is  the  concentration  or  mass  of  the 
reacting  constituent  which  is  transformed  which  is  increased,  or 
whether  it  is  the  concentration  or  the  nature  of  the  enzyme  which 

1 J.  Loeb,  J.  Gen.  Physlol.  1,  81  (1918)  ;  2,  297,  651  (1920)  ;  Science  54, 
521  (1921). 

2  Cf.  J.  Loeb  ;  "Forced  Movements,  Tropisms  and  Animal  Conduct."  Mono- 
graphs on  Experimental  Biology.  J.  B.  Lippincott  Company,  Philadelphia  and 
London,  1918.  J.  Loeb  and  J.  H.  Northrop,  Proc.  Nat.  Acad.  Sci.  3f  539  (1917). 


134  CATALYTIC  ACTION 

is  changed.  The  data  available  at  present  do  not  permit  of  a 
decision  on  this  point  being  reached. 

Some  of  the  further  experimental  studies  which  bear  upon 
the  problems  discussed  in  the  earlier  pages  of  this  chapter  will 
now  be  taken  up.  One  of  the  main  questions  involved  includes 
the  development  or  production  of  enzymes  in  and  by  living  or- 
ganisms and  the  effect  on  these  enzymes  of  changes  in  the  me- 
dium or  environment.  During  the  growth  of  higher  plants  and 
animals  the  characters  of  the  enzymes  frequently  change  as  the 
organisms  develop.  These  changes  are  fundamentally  of  the 
nature  of  the  changes  under  discussion.  The  conditions  under 
which  these  changes  occur  do  not,  however,  permit  of  ready  alter- 
ation experimentally  of  the  external  conditions  or  environment. 
The  possible  changes  in  enzyme  actions  resulting  are  of  great 
interest,  but  because  of  lack  of  experimental  control  in  most  cases 
are  not  as  satisfactory  for  a  discussion  of  this  sort.  They  will 
not,  therefore,  be  spoken  of  farther  in  this  connection.  More 
satisfactory  studies  of  the  desired  nature  have,  however,  been 
begun  with  some  of  the  lower  organisms  such  as  yeasts  and  bac- 
teria. By  suitable  changes  in  the  nutrient  media,  the  amounts  of 
certain  enzymes  have  been  increased  to  a  considerable  extent. 
Reference  may  be  made  in  this  connection  especially  to  the 
studies  of  Euler  and  his  co-workers.1  It  was  found  that  yeast 
(definite  strains  from  beer  fermentations)  showed  a  maximum 
growth  as  well  as  a  maximum  formation  of  sucrase  at  pH  5-6.  In 
general,  conditions  which  favored  the  former  favored  the  latter 
as  well,  although  this  was  only  a  rough  parallelism.  However, 
a  more  recent  study  2  showed  that  the  growth  stimulant  prob- 
ably was  not  the  same  substance  which  increased  the  rate  of 
sucrase  formation.  A  partial  separation  of  the  two  substances 
was  effected  by  extraction  of  the  former  with  benzene,  by  adsorp- 
tion with  fuller's  earth,  and  by  precipitation  with  phosphotung- 
stic  acid. 

An  extended  investigation  of  the  formation  of  urease  by  bac- 
teria was  published  by  M.  Jacoby.3  He  followed  the  change  in 
amounts  of  urease  produced  in  urease  forming  bacteria  by  the 

1  For  a  summary  of  this  work  as  well  as  for  references  to  the  original  lit- 
erature   cf.    H.    Euler,    "Chemie    der    Enzyme,    1.    Teil.    Allgemeine    Chemie    der 
Enzyme,"  1920.     Pp.  291-6. 

2  E.  W.  Miller,  J.  Biol.  Uhem.  Jt8,  329   (1921). 
3M.  Jacoby,  Biochem.  Z.  79,  35   (1917). 


CHEMICAL  INTERPRETATION  OF  LIFE  PROCESSES  135 

addition  of  a  number  of  organic  substances  to  the  nutrient  mix- 
ture. A  number  of  these  substances,  including  hexoses  and  three 
carbon  sugars,  increased  the  amounts  of  urease  considerably, 
while  a  larger  number,  including  various  carbohydrates,  had  no 
apparent  effect.  It  was  also  found  by  Euler  and  Asarnoj  that 
the  addition  of  starch  to  the  nutrient  solution  increased  the  pro- 
duction of  amylase  by  Aspergillus  niger.1 

The  increases  in  enzyme  concentrations  described,  and  others 
which  might  be  quoted,  brought  about  by  suitable  alterations  of 
the  media,  are  of  importance,  but  the  changes  which  have  been 
called  "acclimatization"  appear  to  be  of  greater  interest  and 
significance. 

The  ability  of  a  yeast  to  ferment  a  given  sugar  depends  to  a 
certain  extent  upon  its  previous  history.  Thus,  it  has  been  pos- 
sible to  increase  the  galactose  fermenting  action  of  a  yeast  to  a 
considerable  extent.  One  of  the  methods  of  doing  this  may  be 
outlined  briefly  as  follows:  Two  grams  of  washed  and  pressed 
yeast  were  mixed  with  200  cubic  centimeters  sterilized  nutrient 
solution  containing  the  ordinary  salts  together  with  4  grams 
asparagine  and  20  grams  of  the  carbohydrate  in  question,  and 
allowed  to  stand  at  ordinary  temperatures  for  different  lengths 
of  time.  The  solution  was  then  decanted  from  the  yeast,  the 
latter  washed,  and  dried  on  clay.  The  fermenting  actions  on 
different  carbohydrates,  calculated  to  the  common  basis  of  dried 
yeast,  were  determined  by  measurement  of  the  carbon  dioxide 
evolved  in  a  definite  period  of  time.  One  series  of  experiments 
in  which  a  top  yeast  was  acclimatized  to  galactose,  gave  for  the 
original  fermenting  action  a  ratio  of  1:50  for  the  actions  on 
galactose  and  on  sucrose,  while  after  treatment  with  galactose, 
the  ratio  was  found  to  be  1:6.5.  An  eight  fold  increase  in  fer- 
menting action  on  galactose  had  been  obtained.2  With  bottom 
yeast,  the  final  ratio  1:2.4  was  obtained.  The  juice  obtained 
from  acclimatized  yeast  was  also  found  to  ferment  galactose.3 
The  question  of  acclimatization  is  treated  at  some  length  by  A. 
Harden  in  his  monograph  on  "Alcoholic  Fermentation."4 

1H.  Euler  and  S.  Asarnoj,  Fermentforschung  3,  318  (1920).  The  sucrase 
activity  of  Aspergillus  niger  was  found  to  be  increased  about  30%  by  the  addi- 
tion of  peptone. 

2  H.  Euler,  I.  Laurin,  and  A.  Pettersson,  Biochem.  Z.   114,  277    (1921). 

3  A.  Harden  and  R.  V.   Norris,  Proc.  Roy.  Soc.  London   (B)   82,  645    (1910). 
*Pp.  109-112. 


136  CATALYTIC  ACTION 

These  results  show  in  every  case  an  increased  action  by  an 
enzyme  which,  however,  was  present  before  the  treatment  even  if 
only  to  a  very  small  extent.  In  principle,  such  an  increase  is  of 
as  great  significance  as  the  formation  of  an  entirely  new  enzyme 
not  present  before,  but  even  so,  it  is  of  interest  to  note  that  this 
problem  of  enzyme  formation  also  appears  to  have  been  solved. 
"The  question,  if,  and  under  what  conditions  it  would  be  pos- 
sible, by  chemical  or  physical  influences  on  the  cell  to  produce 
enzymes  which  otherwise  are  foreign  to  the  given  microorgan- 
isms, is  still  open;  it  is  difficult  to  answer  directly,  as  it  is  always 
possible  that  a  trace  of  the  corresponding  enzyme  might  be  pres- 
ent, so  that  a  new  formation  did  not  occur.  The  writer  (Euler) , 
starting  with  the  view  that  a  marked  and  reproducible  influence 
could  be  obtained  by  symbiosis,  sought  for  and  found  the  forma- 
tion of  amylase  in  microorganisms,  in  which  previously  by  the 
same  methods,  no  indication  of  the  presence  of  this  enzyme  could 
be  obtained."  x 

The  changes  in  bacterial  cultures  which  may  be  brought  about 
by  changes  in  the  media  in  which  they  grow  may  properly  be 
included  here.  The  question  of  growth  in  purely  synthetic  media 
is  also  involved.  Careful  studies,  qualitative  and  quantitative, 
of  the  actions  of  enzymes  obtained  from  bacteria  grown  under 
various  conditions  are  not  common.  It  may  be  of  greater  interest 
to  present  in  this  connection  extracts  from  an  address  2  by  a  lead- 
ing worker  in  this  field  as  representing  the  present  status  of  the 
problem. 

"The  diphtheria  bacillus,  grown  in  appropriate  nitrogenous  media  con- 
taining no  utilizable  sugar  or  an  amount  of  sugar  which  the  organism  can 
use  up  without  inhibiting  its  further  activity,  will  produce  a  very  potent 
soluble  toxin.  If  the  same  organism  is  grown  in  the  same  nitrogenous 
medium,  to  which  is  added  sufficient  utilizable  sugar  so  that  the  organism 
cannot  use  it  up,  it  will  be  found  that  no  toxin  whatever  will  appear  in 
the  culture.  Similarly,  the  colon  bacillus  grown  in  sugar-free  nitrogenous 
media  will  produce  indol,  ammonia,  hydrogen  sulphide  and  other  products 
indicative  of  the  break-down  of  protein;  but  the  same  organism  grown  in 
the  same  nitrogenous  medium,  to  which  utilizable  sugars  are  added,  will 
not  contain  any  of  these  products  indicative  of  protein  break-down.  On 
the  contrary,  the  characteristic  products  are  organic  acids,  carbon  dioxide 
and  hydrogen. 

"The  proteus  bacillus  is  one  of  a  considerable  number  of  bacteria  which 
produce  a  soluble  enzyme  in  sugar-free  gelatin,  which  liquefies  this  protein. 
The  sterile  filtrate  of  such  a  liquefied  gelatin  culture  will  contain  enough 

1  H.  Euler,  "Allgemeine  Chemie  der  Enzyme,"  p.  296. 

2  "Recent    Developments    in    Intestinal    Bacteriology,"    by    A.    I.     Kendall. 
Published  in  Am.  J.  Med.  8ci.  156,  157   (1918). 


CHEMICAL  INTERPRETATION  OF  LIFE  PROCESSES  137 

free  enzyme  to  liquefy  an  additional  amount  of  gelatin.  The  addition  of 
a  small  amount  of  utilizable  sugar  to  gelatin  cultures  of  the  proteus  bacillus 
will  prevent  temporarily  the  formation  of  this  enzyme  in  an  active  state 
and  the  addition  of  a  considerable  amount  will  permanently  prevent  the 
development  of  the  gelatin-liquefying  enzyme.  It  is  worthy  of  note  that 
the  mature,  soluble,  gelatin-liquefying  enzyme,  freed  from  bacteria  by 
filtration  through  a  porcelain  filter,  will  liquefy  sterile  gelatin  containing 
the  same  sugar  which  in  culture  prevented  the  formation  of  the  enzyme, 
clearly  suggesting  that  the  presence  of  utilizable  carbohydrate  prevented 
the  elaboration  of  the  enzyme,  but  had  no  effect  upon  the  action  of  the 
enzyme  once  it  was  excreted  in  an  active  state. 

"The  relation  between  the  factors  which  form  the  production  of  this 
proteolytic  enzyme  and  those  which  are  associated  with  its  action  when  it 
is  mature  is  even  more  striking.  Chemical  examination  of  cultures  (free 
from  utilizable  carbohydrate)  in  which  the  enzyme  develops  in  increasing 
proportions  shows  the  presence  of  increasing  amounts  of  ammonia.  A 
similar  examination  of  sterile  gelatin  mixtures  (with  or  without  carbo- 
hydrate), in  which  the  germ-free  enzyme  induces  liquefaction,  fails  to  reveal 
any  increase  whatsoever  in  ammonia.  Apparently  the  activity  of  the 
enzyme  is  independent  of  ammonia  formation.  Nevertheless,  ammonia 
formation  invariably  occurs  when  the  enzyme  is  formed  by  the  bacillus. 
The  facts  presented  would  seem  to  justify  the  following  deductions:  The 
soluble  gelatin-liquefying  enzyme  of  Bacillus  proteus  is  formed  when  the 
organism  utilizes  protein  for  its  energy.  It  is  not  formed  in  an  active 
state  when  utilizable  carbohydrates  are  continually'  available  for  energy. 
(Small  amounts  of  carbohydrate  insufficient  to  prevent  the  development 
of  the  organism  have  been  shown  in  similar  experiments  to  inhibit  enzyme 
formation  until  the  carbohydrate  is  used  up.  Acid  within  the  limits  of 
tolerance  of  the  organism  does  not  inhibit  enzyme  formation.)  The  func- 
tion of  the  enzyme  is  to  prepare  protein  for  assimilation  by  the  organism, 
as  the  enzymes  of  the  alimentary  canal  of  man  prepare  proteins  for  assimi- 
lation. Ammonia  formation  is  a  measure  of  the  deaminization  of  the 
assimilated  protein  fragment  and  not  a  concomitant  feature  of  the  action 
of  the  enzyme  per  se.  The  ammonia  is  'bacterial  urea.'  Other  organisms 
which  form  similar  soluble  proteolytic  enzymes  react  in  like  manner  to  the 
presence  of  utilizable  carbohydrates. 

"The  protective  or  sparing  action  which  utilizable  carbohydrate  exhibits 
for  protein  or  protein  derivatives  in  cultures  of  bacteria  has  its  counterpart 
in  higher  organisms.  The  underlying  principle  is  apparently  analogous  to 
that  in  man:  Physiologists  have  long  stated  that  the  oxidation  of  carbo- 
hydrates protects  the  protein  of  the  body.  Carbohydrates,  in  other  words, 
are  protein  sparers,  and,  as  Howell  has  aptly  stated  it,  'as  the  carbohydrate 
food  is  increased  the-  protein  food  may  be  diminished  down  to  a  certain 
irreducible  minimum,  which  is  probably  the  amount  necessary  for  the 
reconstruction  of  new  tissue.' 

"The  sparing  action  of  utilizable  carbohydrate  for  protein  in  cultures 
of  bacteria  has  a  deeper  significance  than  has  been  realized  hitherto.  The 
diphtheria,  colon  and  proteus  bacillus,  to  mention  merely  those  referred 
to  above,  form  widely  different  products  as  the  result  of  their  growth  in 
nitrogenous  media  from  which  utilizable  carbohydrates  are  excluded.  The 
same  is  equally  true  for  typhoid,  paratyphoid,  dysentery,  hemorrhagic  sep- 
ticemic,  tetanus,  "gas,"  symptomatic  anthrax  and  many  other  pathogenic 
bacilli,  cholera  and  other  vibrios  and  a  large  number  of  saprophytic  or- 
ganisms as  well.  Stated  differently,  it  is  positively  known  that  diphtheria 
and  tetanus  bacilli  form  highly  specific  soluble  toxins  as  they  develop  in 
protein  media.  They  form  innocuous  fermentation  products,  chiefly  acids, 
as  lactic  and  acetic,  from  the  same  media  to  which  utilizable  carbohydrates 


138  CATALYTIC  ACTION 

are  added.  That  which  makes  diphtheria  and  tetanus  bacilli  formidable, 
in  other  words,  is  apparently  inseparably  associated  with  their  growth  in 
nitrogenous  but  carbohydrate-free  media.  The  antithesis  of  this  speci- 
ficity of  products  developed  in  nitrogenous,  sugar-free  media  is  manifested 
in  the  remarkable  similarity  of  products  formed  in  the  same  media  which 
contain  utilizable  carbohydrate  in  addition.  The  nitrogenous  product* 
formed  by  typhoid,  dysentery,  cholera  and  other  pathogenic  organisms  are 
unknown  as  yet,  but  the  simple  addition  of  utilizable  carbohydrate  to 
cultural  media  in  which  they  are  grown  causes  them  to  produce  fermen- 
tation products,  as  lactic  acid,  precisely  as  the  diphtheria  bacillus  does 
under  similar  circumstances.  The  chemical  products  formed  by  these  bac- 
teria in  the  presence  of  utilizable  carbohydrate  are  potentially  those  pro- 
duced by  the  Bulgarian  bacillus;  that  is  to  say,  a  large  number  of  bacteria 
pathogenic  for  man  become  potentially  lactic  acid  bacilli  when  they  are 
grown  in  fermentation  media,  and  from  this  point  of  view,  therefore,  their 
specificity  of  action  is  inseparably  associated  with  the  utilization  of  protein 
for  energy. 

"Typhoid,  dysentery,  and  diphtheria  bacilli  do  not  ferment  lactose.  If 
milk  contained  dextrose  in  place  of  lactose,  or  if  at  least  0.5  per  cent  of 
dextrose  were  added  to  it,  these  organisms  would  produce  sour  milk  as 
the  result  of  their  growth  in  it  and  this  sour  milk  would  not  differ  quali- 
tatively from  that  produced  by  the  Bulgarian  bacillus.  Furthermore,  so 
long  as  the  bacteria  were  confronted  with  this  dextrose  they  would  continue 
to  make  sour  milk  until  the  acidity  reached  a  degree  incompatible  with 
their  further  growth.  If  the  utilizable  carbohydrate  were  removed,  of 
course  they  would  act  once  more  upon  the  protein." 

In  speaking  of  intestinal  bacteria,  Kendall  stated: 

"There  appears  to  be  an  intimate  relationship  between  the  character 
of  the  diet  and  the  nature  of  the  intestinal  flora.  This  relationship,  bac- 
terially  considered,  is  manifested  by  an  adaptive  intestinal  acclimatization 
of  fairly  definite  types  of  bacteria.  Changes  in  the  diet,  if  prolonged,  tend 
to  change  the  types  of  bacteria.  A  change  in  the  products  of  metabolism 
of  intestinal  bacteria  is  also  induced,  depending  upon  the  presence  or  ab- 
sence of  carbohydrate.  .  .  .  Bacteria  which  are  normally  acclimatized  do 
not  produce  metabolic  products  widely  at  variance  with  the  well-being  of 
the  host.  .  .  .  Bacterial  acclimatization  and  adaptation  is  the  resultant  of 
complex  reciprocal  activities  between  host  and  parasite." 

It  will  hardly  be  necessary  to  enter  farther  into  this  question 
in  the  present  connection.  Changes  in  bacteria  brought  about 
by  differences  in  nutrient  media  are  clearly  recognized.  These 
changes  have  been  shown  experimentally  to  be  accompanied  by 
changes  in  enzyme  actions  in  some  cases.  Undoubtedly,  further 
study  will  show  the  connection  with  enzymes  and  their  actions 
in  all  cases  of  change  in  bacterial  nature.  It  should  be  possible 
with  the  means*  at  hand  and  the  data  already  available  to  carry 
out  conclusive  studies  of  this  kind.  In  fact,  bacterial  enzymes, 
obtained  from  bacterial  cultures  after  using  various  media,  offer 
an  extremely  fertile  field  for  the  study  of  the  relations  between 
the  formation  and  properties  of  enzymes  and  the  chemical  and 
biological  mechanism  of  life  processes. 


CHEMICAL  INTERPRETATION  OF  LIFE  PROCESSES 

The  views  here  developed  are  based  upon  the  following  suc- 
cessive generalizations:  (a)  General  (addition  theory)  of  chemi- 
cal reactions;  (b)  Catalytic  reactions  as  a  group  of  chemical 
reactions;  (c)  Enzyme  reactions  as  a  group  of  catalytic  reactions; 
(d)  Life  processes  interpreted  as  controlled  essentially  by  en- 
zymes. The  application  of  these  views  might  obviously  be  car- 
ried over  to  other  branches  of  science,  explanations  developed 
based  upon  these  fundamental  conceptions,  and  evidence  brought 
forward  to  support  the  explanations. 

One  more  feature  of  the  problem  of  living  matter  may  be 
mentioned.  The  production  of  enzymes  and  their  actions  were 
considered  to  be  the  essence  of  living  matter.  Enzyme  actions 
have  been  shown  to  be  chemical  reactions  fundamentally.  En- 
zyme production  then  forms  the  chemical  counterpart  of  the 
production  of  life.  Reasoning  from  the  basis  of  the  chemical 
views  which  have  been  presented,  it  may  be  said  that  either  the 
life  process  is  created  or  begun  continuously  in  and  to  form  living 
matter,  every  new  formation  of  enzyme  representing  such  a  cre- 
ation or  beginning,  or  it  is  not  created  at  all,  the  production  of 
new  enzyme  material  being  a  part  of  the  chemical  change  or  part 
of  the  products  formed  by  the  action  of  the  old  enzyme  on  the 
available  material.  Going  back  one  step  farther,  according  to 
the  latter  hypothesis,  a  separate  causation  of  life  need  not  be 
sought.  Life  originally  would  then  have  been  the  chance  result 
of  certain  catalytic  actions  (later  to  be  known  as  enzyme  ac- 
tions) on  suitable  material,  which  formed  continuously  new  cata- 
lytic (or  enzyme)  substances  (auto-catalytic  in  the  present  termi- 
nology), resulting  in  the  cycles  of  changes  described  in  this 
chapter.  The  two  possibilities  present  themselves  in  a  compara- 
tively simple  manner;  either  the  continuous  ever-present  crea- 
tion of  life  as  long  as  living  matter  exists  or  has  existed,  in  place 
of  the  one  creation  (or  possibly  several  creations)  at  some  remote 
period  in  the  past,  or  no  creation  in  the  extra-scientific  sense  at 
any  time,  but  a  continuous  cycle  of  changes  which  can  be  in- 
terpreted on  the  basis  of  definite  well-known  chemical  principles, 
in  which  the  continuity,  change,  development,  evolution,  and 
even  the  beginning,  require  no  new  fundamental  concepts  of 
science  or  philosophy. 


Chapter  VIII. 
Contact  Catalysis. 

The  term  "contact  catalysis"  has  been  used  at  various  times 
to  include  catalytic  reactions  in  which  the  substances  involved 
occur  in  two  or  more  phases.  In  order  to  discuss  such  reactions 
with  as  much  understanding  as  possible,  it  is  well  to  consider  the 
significance  of  the  definitions,  the  limitations  of  the  phenomena, 
and  their  relations  to  other  chemical  and  physical  changes,  be- 
fore speaking  of  a  number  of  individual  reactions. 

"Contact"  reactions  or  processes  are  stated  as  a  rule  to  in- 
clude chemical  reactions  which  take  place  in  heterogeneous  sys- 
tems. In  the  present  connection,  in  the  consideration  of  reac- 
tions taking  place  in  heterogeneous  systems,  it  will  be  postu- 
lated that  the  substances  taking  part  in  the  reaction  must  be 
present  in  two  or  more  phases  before  the  chemical  change  has 
occurred.  Each  substance  need  not  be  present  in  two  phases, 
but  substances  existing  initially  in  at  least  two  phases  must 
react.  They  may  be  present  in  one,  two,  or  any  number  of 
phases  after  the  reaction  has  occurred,  but  this  is  of  no  direct 
importance  in  the  classification  of  the  reactions.  This  view  at 
once  excludes  reactions  in  which  gases  are  evolved  or  precipitates 
formed  in  changes  taking  place  in  solutions  initially  homogeneous, 
etc.,  and,  in  fact,  all  reactions  in  which  the  substances  are  origi- 
nally present  in  one  phase  or  as  a  homogeneous  system.  Also, 
for  the  present,  reactions  in  which  radiations  of  various  types  are 
known  to  play  a  prominent  part  will  be  excluded.  This  appears 
to  be  advisable  as  the  relations  of  such  reactions  to  those  in 
which  radiation  influences  are  not  so  apparent  are  not  as  yet 
well  developed.  In  view  of  the  extensive  experimental  work 
which  is  in  progress  along  these  lines,  it  may  confidently  be  ex- 
pected that,  in  the  near  future,  relations  will  have  been  devel- 
oped to  permit  of  satisfactory  classification  of  radiation  reac- 

140 


CONTACT  CATALYSIS  141 

tions.  At  the  same  time,  an  increased  knowledge  and  better 
understanding  of  all  chemical  reactions  is  bound  to  result  from 
the  study  of  phenomena  as  fundamental  for  chemical  change  as 
radiation. 

The  reactions  which  take  place  in  heterogeneous  systems  in 
which  the  substances  are  present  initially  in  two  or  more  phases 
are,  therefore,  to  be  termed  "contact  reactions."  The  reactions 
which  may  be  included  under  the  term  "contact  catalysis"  bear 
to  contact  reactions  exactly  the  same  relations  which  catalytic 
reactions  in  general  bear  to  chemical  reactions  in  general.  What- 
ever definition  is  assumed  to  apply  to  one  set  of  relations  may 
be  carried  over  to  the  other  set  of  relations.  It  was  pointed  out. 
in  the  earlier  chapters  of  this  book  that  the  writer  prefers  to 
consider  a  catalytic  reaction  as  a  reaction  in  which  the  chemical 
composition  of  one  of  the  final  products  of  the  reaction  is  the 
same  as  that  of  one  of  the  initial  substances.  This  definition  is 
evidently  directly  applicable  to  contact  catalytic  reactions.  The 
fact  that  a  catalytic  substance  has  taken  part  in  the  contact 
reaction  may  be  made  apparent  as  with  other  reactions  by  a 
change  in  the  velocity  of  the  reaction,  or  by  a  change  (complete 
or  partial)  in  the  products  obtained.  The  use  of  a  different  defi- 
nition of  catalytic  action,  however,  will  not  affect  the  view  of 
the  relations  outlined  here  between  contact  catalytic  reactions, 
contact  reactions  in  general,  and  chemical  reactions  as  a  whole. 
This  classification  will  hold  without  modification  for  any  chem- 
ical theory  of  catalysis  which  has  been  suggested,  but  if  the 
term  catalysis  is  to  be  retained  in  chemical  literature,  the  writer 
prefers  the  definition  first  given. 

Since,  in  heterogeneous  systems,  reactions  between  substances 
present  in  different  phases  take  place  at  or  near  the  surfaces  of 
the  phases,  a  review  of  some  of  the  recent  work  bearing  upon  the 
condition  and  behavior  of  substances  at  interfaces  will  be  given. 
It  is  only  very  recently  that  light  has  been  thrown  on  the  chem- 
ical states  of  substances  under  such  conditions.  Before  the 
studies  which  will  be  spoken  of  were  made,  it  was  necessary  to 
take  refuge  in  general  statements  based  upon  inconclusive  ex- 
perimental material  in  describing  the  conditions  and  the  actions 
at  such  interfaces. 

The  phenomena  which  are  included  in  the  term  "adsorption" 


142  CATALYTIC  ACTION 

must  be  taken  up  in  connection  with  the  relations  to  be  described. 
Adsorption  may  be  defined  as  the  difference  in  the  composition 
of  the  surface  layer  at  the  contact  of  two  phases  and  the  compo- 
sitions in  the  main  bodies  of  these  phases. 

The  recent  advances  in  the  knowledge  of  the  conditions  of 
surfaces,  of  reactions  taking  place  at  interfaces,  and  of  adsorp- 
tion, are  due  in  the  main  to  the  work  of  I.  Langmuir  and  of  W.  D. 
Harkins. 

The  striking  feature  of  the  advance  made  by  Langmuir  1  in 
the  treatment  of  the  conditions  at  interfaces  is  the  emphasis 
which  he  placed  upon  the  chemical  nature  of  the  forces  involved 
in  adsorption,  surface  tension,  evaporation,  crystallization,  etc., 
these  forces  being  considered  not  to  differ  "in  any  essential  re- 
spect from  the  forces  which  cause  the  formation  of  typical  chemi- 
cal compounds."  2  These  same  forces  are  active  in  solids  and 
liquids  which  are  considered  in  any  given  case  to  consist  of  one 
large  molecule,  the  atoms  of  which  are  held  together  by  chemical 
forces.  Langmuir  is  definite  with  regard  to  the  fact  that  these 
chemical  forces  which  cause  the  various  phenomena  are  essen- 
tially the  same.  At  the  same  time,  a  point  must  be  mentioned 
which  will  be  taken  up  again  later  in  this  chapter.  In  attempt- 
ing to  define  these  chemical  forces  in  explicit  terms,  Langmuir 
did  not  present  as  satisfactory  a  picture.  In  various  places  he 
spoke  of  chemical  forces  acting  by  means  of  "primary"  valences 
and  "secondary"  valences.  He  gave  the  characteristics  of  these 
forces  in  discussing  the  interatomic  and  intermolecular  forces 
involved  in  the  structure  of  matter,3  but  to  the  writer  there  ap- 
pears to  be  no  essential  difference  between  these  two  forms  of 
valence  as  defined,  although  it  must  be  added  that  the  use  he 
made  of  the  two  conceptions  indicated  definite  differences,  as  will 
be  pointed  out  later.  This  is,  however,  a  minor  factor  in  these 
considerations.  Langmuir  made  clear  the  fact  that  the  forces 
are  chemical  and  not  physical  in  nature,  and  this  is  the  main 
consideration  in  the  present  discussion. 

The  first  point  of  interest  in  Langmuir's  treatment  relates  to 
the  question  of  the  nature  of  the  surfaces  of  solids  and  liquids 

1I.  Langmuir,  Met.  Chem.  Eng.  15,  469  (1916)  ;  Jour.  Amer.  Chem.  Soc.  38, 
2221  (1916)  ;  39,  1848  (1917)  ;  40,  1361  (1918)  ;  Chem.  News  m,  225,  237 
(1921). 

2  Jour.  Amer.  Chem.  Soc.  39,  1901   (1917). 

3  Jour.  Amer.  Chem.  Soc.  39,  1853  (1917). 


CONTACT  CATALYSIS*  143 

in  contact  with  gases.  It  was  pointed  out  that  there  is  not  a 
transition  layer  either  for  solids  or  liquids  "in  which  the  density 
varies  by  continuous  gradations  from  that  of  the  solid  to  that 
of  the  surrounding  gas  or  vapor."  x  The  transition  was  consid- 
ered to  be  abrupt,  and  for  solids  to  be  accompanied  by  a  greater 
density  of  the  packing  of  the  atoms  in  the  surface  layer  than 
in  the  body  of  the  solid.  Following  the  views  of  the  arrange- 
ments of  atoms  in  crystals  developed  by  means  of  x-ray  studies, 
it  was  shown  that  "the  surface  must  be  looked  upon  as  a  sort  of 
checkerboard  containing  a  definite  number  of  atoms,  of  definite 
kinds,  arranged  in  a  plane  lattice  formation.  The  space  between 
and  immediately  above  (away  from  the  interior)  these  atoms  is 
surrounded  by  a  field  of  electromagnetic  force  more  intense  than 
that  between  the  atoms  inside  the  crystals."2  "The  only  essen- 
tial differences  between  liquids  and  solids  seem  to  lie  in  the 
mobility  of  liquids  and  in  those  properties  which  are  directly 
dependent  on  mobility.  As  a  result  of  this  mobility,  the  definite 
lattice  arrangement  of  the  atoms  of  solids  gives  way  to  the 
irregular  arrangements  characteristic  of  liquids." 3  The  atoms 
are  all  assumed  to  be  combined  as  one  large  molecule  in  a  liquid, 
the  mobility  being  due  to  some  sort  of  tautomerism.  It  is  ob- 
viously impracticable  to  enter  in  detail  into  the  evidence  which 
was  presented  by  Langmuir  in  connection  with  these  deductions. 
The  actions  of  the  solid  and  liquid  surfaces  in  combining  with 
substances  present  initially  in  other  .phases  (so-called  adsorp- 
tion) was  treated  by  Langmuir  from  the  point  of  view  just  out- 
lined. The  forces  acting  are  chemical  in  character,  and,  because 
of  the  stronger  electromagnetic  fields  existing  at  surfaces,  are 
perhaps  more  intense  than  under  different  conditions.  Whether 
these  chemical  forces  represent  so-called  primary  or  secondary 
valences  is  immaterial.  Gases  impinging  upon  the  surfaces  of 
solids  or  liquids  did  not  in  general  rebound  elastically,  but  con- 
densed on  the  surface  being  held  by  the  field  of  force  of  the 
surface  atoms.  The  relative  lengths  of  time  these  gases  are  held 
before  evaporating  determines  the  adsorption.  The  surface  would 
become  chemically  saturated  by  a  layer  one  molecule  thick. 
Since  the  forces  act  between  the  atoms  at  the  surface  and  par- 

1  Jour.  Amer.  Chem.  Soc.  38,  2249  (1916). 

2  Jour.  Amer.  Chem.  Soc.  38,  2249   (1916). 
*Jour.  Amer.  Chem.  Soc.  39,  1857  (1917). 


144 


CATALYTIC  ACTION 


ticular  groups  or  atoms  of  the  adsorbed  molecules,  the  adsorbed 
molecules  usually  orient  themselves  in  definite  ways  in  the  sur- 
face layer.  Thus,  oxygen  adsorbed  on  a  carbon  filament  is 
formulated  as  follows: 


O        0       0  oxygen  layer 

6  B  B 
'\y\/ 

c     c     c     c 

/\/\/\. 

c     c     c     c 

Similarly  for  carbon  monoxide  and  tungsten: 


body  of  filament 


00000 


w    w    w    w 
w    w    w    w 


adsorbed  layer 


body  of  filament 


Evidence  based  in  the  first  instance  upon  the  electron  emission 
from  heated  metals  showed  that  the  adsorbed  film  was  held  to 
the  surface  by  chemical  forces,  and  consisted  in  all  probability 
of  a  single  layer  of  molecules;  at  most  of  a  very  few  layers.  It 
was  found,  for  example,  that  substances  which  reacted  with 
tungsten  lowered  the  electron  emission  from  it;  inert  gases  ,did 
not.  This  view  was  then  extended  and  applied  to  gases  adsorbed 
on  solids  at  ordinary  pressures,  and  to  adsorption  in  surface 
layers  of  liquids.  It  was  pointed  out  that  with  gases  and  solids 
"the  term  adsorption  should  be  restricted  to  gas  taken  up  on  a 
surface  without  any  penetration  of  the  gas  molecules  between  the 
atoms  or  molecules  on  the  solid  surface."  x  In  a  subsequent 
paper  on  "The  Adsorption  of  Gases  on  Plane  Surfaces  of  Glass, 
Mica  and  Platinum,"  2  the  experimental  work  was  extended  and 
the  theoretical  explanations  of  the  phenomena  confirmed.  It  was 
pointed  out  that  in  the  study  of  adsorbed  films,  much  of  the  work 
described  in  the  past  involved  the  use  of  porous  materials  such 

*Jour.  Amer.  Ghent.  Soc.  38,  2284   (1916). 
2/&id.  40,  1361  (1918). 


CONTACT  CATALYSIS  145 

as  charcoal  which  made  it  difficult  to  determine  the  extent  of  the 
surface,  that  frequently  because  of  the  method  of  packing  sub- 
stances enormous  numbers  of  capillary  spaces  might  be  formed 
in  which  with  saturated  or  nearly  saturated  vapors,  actual  con- 
densation of  the  liquid  in  the  capillary  spaces  would  occur,  and 
that  at  times  substances  were  used  which  actually  dissolved  the 
gas  or  vapor  thought  to  be  adsorbed,  as,  for  example,  water  vapor 
in  glass.  The  use  of  glass,  mica,  and  platinum  with  a  number 
of  gases  at  low  as  well  as  at  high  pressures  overcame  these  diffi- 
culties and  made  satisfactory  quantitative  measurements  pos- 
sible. The  results  obtained  with  these  substances  may  be 
quoted.1 

"It  was  found  that  at  100  bars'  pressure  the  amounts  of  oxygen,  nitrogen, 
hydrogen,  carbon  monoxide,  carbon  dioxide  and  argon  adsorbed  on  glass 
or  mica  at  room  temperature  were  not  measurable,  although  if  0.0002  of 
the  surface  had  been  covered  by  a  layer  one  molecule  deep  it  could  have 
been  readily  detected.  On  cooling  these  surfaces  to  liquid  air  temperature 
the  surfaces  became  covered  with  a  monomolecular  layer  to  the  extent  of 
1  to  10  per  cent  and  at  100  bars'  pressure  they  seemed  nearly  saturated. 
The  relatve  amounts  of  different  gases  absorbed  were  in  the  same  order  as 
the  boiling  points,  showing  that  the  forces  involved  in  this  adsorption  were 
secondary  valence  forces  such  as  those  involved  in  the  liquefaction  of 
these  gases. 

"With  platinum  surfaces  the  phenomena  were  totally  different.  Even 
at  pressures  below  one  bar  the  surface  adsorbed  hydrogen,  carbon  mon- 
oxide or  oxygen  immediately  to  form  a  layer  covering  the  surface  with 
a  layer  approximately  one  molecule  (or  atom)  deep.  These  films  could 
not  be  driven  off  by  heating  to  360°,  but  could  be  made  to  displace  one 
another.  The  surfaces  were  wholly  saturated  at  a  few  bars'  pressure,  and 
no  increase  in  adsorption  could  be  noted  by  raising  the  pressure  to  200 
bars.  These  films  are  evidently  held  to  the  platinum  by  primary  valence 
forces. 

"With  the  platinum  at  liquid  air  temperature  the  gases  are  first  adsorbed 
by  secondary  valence  forces,  because  when  the  temperature  is  raised  to 
room  temperature  the  gas  first  comes  off  the  surface,  and  then  at  a  tem- 
perature somewhat  below  room  temperature  goes  back  again  onto  the 
surface. 

"In  no  case,  however,  was  any  adsorption  noted  which  corresponded  to 
a  layer  more  than  one  moleclue  deep." 

The  different  conditions  of  adsorption  were  considered  theo- 
retically, and  mathematical  expressions  developed  which  readily 
accounted  for  the  adsorption  formula  usually  employed. 

The  properties  of  the  surface  layers  of  liquids  were  treated 
by  the  same  methods  by  Langmuir.  His  views,  confirmed  by  all 
subsequent  work,  may  be  quoted  from  his  first  paper  on  the 
subject:  2 

^Jour.  Amer.  Chem.  Soc.  39,  1904-5    (1917). 
2  Met.  Chem.  Eng.  15,  469  (1916). 


146  CATALYTIC  ACTION 

"A  theory  of  surface  tension  is  now  proposed  in  which  the  structure  of 
the  surface  layer  of  atoms  is  regarded  as  the  principal  factor  in  determining 
the  surface  tension  (or  rather  surface  energy)  of  liquids.  This  theory  is 
supported  in  the  most  remarkable  way  by  all  available  published  data  on 
the  surface  tension  of  organic  liquids. 

"According  to  this  theory,  the  group  molecules  of  organic  liquids  arrange 
themselves  in  the  surface  layer  in  such  a  way  that  their  active  portions  are 
drawn  inwards,  leaving  the  least  active  portion  of  the  molecule  to  form 
the  surface  layer.  By  'active  portion'  of  a  molecule  is  meant  a  portion 
which  is  characterized  by  a  strong  stray  field  (residual  valence).  Chemical 
.action  may  be  assumed  to  be  due  to  the  presence  of  electromagnetic  fields 
surrounding  atoms.  Surface  tension  (or  surface  energy)  is  thus  a  measure 
of  the  potential  energy  of  the  electromagnetic  stray  field  which  extends  out 
from  the  surface  layer  of  atoms.  The  molecules  in  the  surface  layer  of  the 
liquid  arrange  themselves  so  that  this  stray  field  is  a  minimum. 

"The  surface  energy  of  a  liquid  is  thus-  not  a  property  of  the  group 
molecules,  but  depends  only  on  the  least  active  portions  of  the  molecules 
and  on  the  manner  in  which  these  are  able  to  arrange  themselves  in  the 
surface  layer. 

"In  liquid  hydrocarbons  of  the  paraffin  series  the  molecules  arrange 
themselves  so  that  the  methyl  groups  (CH3)  at  the  ends  of  the  hydro- 
carbon chains  form  the  surface  layer.  The  surface  layer  is  thus  the  same, 
no  matter  how  long  the  hydrocarbon  chain  may  be.  As  a  matter  of  fact, 
the  surface  energy  of  all  these  many  different  substances  from  hexane  to 
molten  paraffin,  have  substantially  the  same  surface  energy,  namely  46  to 
48  ergs  per  sq.  cm.,  although  the  molecular  weights  differ  greatly. 

"If,  now,  we  consider  the  alcohols  such  as  CH3OH,  C2H5OH,  etc.,  we 
find  that  their  surface  energies  are  practically  identical  with  those  of  the 
hydrocarbons.  The  reason  for  this  is  that  the  surface  layer  in  both  cases 
consists  of  CH3  groups. 

"With  such  substances  as  CH3NO2,  CHsI,  we  find  that  the  surface  energy 
is  much  greater  than  that  of  the  hydrocarbons.  This  is  due  to  the  fact 
that  the  volume  of  the  I  or  the  NO2  is  so  great  that  the  surface  cannot 
be  completely  covered  by  the  CH3  radicals.  The  forcing  apart  of  these 
groups  increases  the  surface  energy. 

"Particularly  interesting  relations  are  found  with  benzene  derivatives. 

"In  benzene  itself,  the  group  molecules  arrange  themselves  so  that  the 
benzene  rings  lie  flat  on  the  surface,  since  the  flat  sides  of  these  rings  are 
the  least  active  portions  of  the  molecules.  The  surface  energy  of  benzene 
is  about  65  ergs  per  square  cm. 

"If,  now,  an  active  group,  such  as  OH,  is  substituted  for  one  of  the 
hydrogens  of  the  benzene  (forming  phenol  or  carbolic  acid)  this  group  is 
drawn  into  the  body  of  the  liquid  tilting  the  benzene  ring  up  on  edge  and 
raising  the  surface  energy  to  about  75  ergs  per  sq.  cm.,  which  corresponds 
to  the  activity  of  the  perimeter  of  the  benzene  ring.  Thus  any  active 
group  strong  enough  to  tilt  the  ring  up  on  edge  raises  the  surface  energy 
to  about  75.  Two  active  groups  side  by  side  (ortho  position)  have  no 
greater  effect  than  one.  But  two  active  groups  opposite  one  another  (para 
position)  cannot  both  go  wholly  below  the  surface,  so  that  the  surface 
energy  then  becomes  abnormally  large  (about  85  in  case  of  p-nitro- 
phenol).  The  substitution  of  methyl  or  ethyl  groups  in  the  benzene  ring 
lowers  the  surface  energy  except  where  an  active  group  in  an  adjacent 
position  draws  these  groups  below  the  surface. 

"Some  of  the  best  evidence  in  support  of  the  new  theory  is  derived 
from  experiments  on  thin  films  of  oil  on  water  or  mercury.  Oleic  acid  on 
water  forms  a  film  one  molecule  deep,  in  which  the  hydrocarbon  chains 
stand  vertically  on  the  water  surface  with  the  COOH  groups  in  contact 
with  the  water. 


CONTACT  CATALYSIS  147 

"Acetic  acid  is  readily  soluble  in  water  because  the  COOH  group  has  a 
strong  secondary  valence  by  which  it  combines  with  water.  Oleic  acid  is 
not  soluble  because  the  affinity  of  the  hydrocarbon  chains  for  water  is  less 
than  their  affinity  for  each  other.  When  oleic  acid  is  placed  on  water  the 
acid  spreads  upon  the  water  because  by  so  doing  the  COOH  can  dissolve 
in  the  water  without  separating  the  hydrocarbon  chains  from  each  other. 

"When  the  surface  on  which  the  acid  spreads  is  sufficiently  large  the 
double  bond  in  the  hydrocarbon  chain  is  also  drawn  down  on  to  the  water 
surface,  so  that  the  area  occupied  is  much  greater  than  in  the  case  of  the 
saturated  fatty  acids. 

"Oils  which  do  not  contain  active  groups,  as  for  example  pure  paraffin 
oil,  do  not  spread  upon  the  surface  of  water." 

The  adsorption  of  liquids  by  solids  was  treated  on  the  basis 
of  Gurvich's  experiments.1  Although  Gurvich  considered  that 
the  adsorption  was  not  due  to  chemical  forces,  Langmuir  2  in- 
terpreted his  results  by  means  of  the  chemical  theory  outlined. 
In  the  adsorption  of  a  liquid  by  a  plane  solid  surface,  the  mole- 
cules of  the  liquid  become  oriented  if  active  groups  are  present. 
There  would  therefore  be  a  tendency  for  a  simple  integral  rela- 
tion to  exist  between  the  number  of  molecules  adsorbed  and  the 
number  of  atoms  exposed  in  the  surface  of  the  solid.  With  a 
crystal,  in  which  the  surface  atoms  are  arranged  in  a  regular 
lattice,  stoichiometric  relations  might  be  expected  to  exist  be- 
tween the  amounts  of  different  liquids  or  gases  needed  to  saturate 
the  surface.  The  number  of  molecules  which  can  be  adsorbed  on 
a  given  surface,  evidently  depends  also  upon  the  configurations 
of  these  molecules.  This  introduces  a  steric  factor,  which  may 
be  of  considerable  importance  in  some  cases.  This  is  true  espe- 
cially of  adsorption  by  porous  bodies,  and  as  the  cavities  become 
smaller,  stoichiometric  relations  would  be  less  frequent. 

It  was  pointed  out  that  the  forces  causing  adsorption  being 
typically  chemical,  exhibit  all  the  great  differences  in  intensity 
and  quality  characteristic  of  chemical  forces.  "Under  certain 
conditions  stoichiometric  relations  should  govern  the  amounts  of 
gas  adsorbed  on  saturated  surfaces.  These  relationships  may 
fail  to  hold  because  of  steric  hindrance  effects  between  the  ad- 
sorbed molecules. 

"Equations  are  developed  which  give  the  relation  between  the 
amount  of  adsorbed  gas  and  the  pressure  and  other  variables 
under  various  assumed  conditions.  No  single  equation  other 
than  purely  thermodynamic  ones  should  be  expected  to  cover  all 

JL.  G.  Gurvich,  J.  Russ.  Phys.-Chem.  Soc.  J,1 ,  805   (1915), 
zjqur.  Amer.  Chem.  Soc.  3$,  1898  (1917), 


148  CATALYTIC  ACTION 

cases  of  adsorption  any  more  than  a  single  equation  should  rep- 
resent equilibrium  pressures  for  all  chemical  reactions."  1 

The  possible  occurrence  of  adsorbed  layers  on  solids  more 
than  one  molecule  in  thickness  was  not  ignored  by  Langmuir, 
but  was  not  considered  to  be  a-dsorption  in  the  strict  sense  of 
the  word.  He  pointed  out  that  such  layers  could  be  formed 
with  nearly  saturated  vapors  where  the  rate  of  evaporation  from 
the  second  layer  of  molecules  is  comparable  with  the  rate  of 
condensation,  and  also  if  the  forces  acting  between  the  first  and 
second  layers  of  adsorbed  molecules  were  greater  than  those 
holding  the  first  layer  to  the  surface. 

Views  similar  to  those  of  Langmuir  relative  to  the  structure 
of  the  surfaces  of  liquids  were  developed  independently  and 
practically  simultaneously  by  Harkins  as  a  result  of  the  study  of 
surface  tensions.2  The  work  done  in  ergs  per  square  centimeter 
when  the  surfaces  of  two  liquids  come  together  to  form  an  inter- 
face was  shown  to  be  characteristic  for  each  class  of  compound  3 
The  data  clearly  brought  out  the  fact  that  the  molecules  of  a 
number  of  liquids  in  contact  with  water  were  oriented  so  that  the 
active  group  (also  called  the  polar  group)  at  the  end  of  a  hydro- 
carbon chain  was  in  contact  with  the  water.  These  "active"  groups 
included  OH,  CHO,  COOH,  double  bond,  triple  bond,  CN,  S03H, 
CONH2,  N02,  NH2,  NCS,  I,  COR,  SH,  etc.  The  surfaces  of 
such  liquids  therefore  show  a  structure.  A  general  law  was  stated 
as  follows:  "If  we  suppose  the  structure  of  the  surface  of  a  liquid 
to  be  at  first  the  same  as  that  of  the  interior  of  the  liquid,  then 
the  actual  surface  is  always  formed  by  the  orientation  of  the 
least  active  portion  of  the  molecule  toward  the  vapor  phase  and 
at  any  surface  or  interface  the  change  which  occurs  is  such  as  to 
make  the  transition  to  the  adjacent  phase  less  abrupt."  It  was 
pointed  out  that  the  fundamental  idea  developed  is  the  same  as 
that  developed  by  Langmuir,  namely,  "that  surface  tension  phe- 
nomena in  general  are  dependent  upon  the  orientation  and  pack- 
ing of  molecules  in  surface  layers,  and  that  the  forces  involved 

1  Jour.  Amer.  Ohem.  Soc.  W,  1401   (1918). 

2  W.  D.  Harkins  and  co-workers,  Jour.  Amer.  Cfhem.  Soc    39,  354,  541   (1917)  ; 
-M,  970    (1919)  ;  42,  700   (1920).     Cf.  also  W.  C.   Reynolds,  J.   Chem.  Soc.   119, 
460,  466  (1921). 

3  In  this  connection,  as  well  as  for  the  first  suggestion  of  a  possihle  orienta- 
tion of  molecules  in  liquid   surfaces,   cf.   W.   B.   Hardy,   Proc.  Roy.  Soc.   London 
(A)  88,  303   (1913).      Cf.  also  the  experimental  study  of  H.  R.  Kruyt  and  C.  F. 
Tan  Duin,  Rcc.  Trav.  Chim.  Pays-Bas  50,  249   (1921). 


CONTACT  CATALYSIS  149 

in  this  action  are  related  to  those  involved  in  solution  and  ad- 
sorption." 

These  views  on  structures  of  surfaces  and  surface  films  indi- 
cate a  general  solution  of  the  problem  of  contact  reactions  as 
well  as  of  contact  catalytic  reactions.  The  formation  of  chem- 
ical compounds  on  the  surface  is  the  first  step  in  such  reactions. 
These  compounds  are  due  to  chemical  forces  (valence  forces). 
In  plane  solid  surfaces,  with  an  adsorbed  film,  the  latter  would 
ordinarily  be  one  atom  or  molecule  thick.  Quantitative  studies 
are  possible  in  such  cases.  With  porous  materials,  or  with  nearly 
saturated  vapors  where  liquefaction  is  possible,  quantitative 
comparisons  cannot  be  carried  out  so  readily.  The  realization 
that  adsorption  compounds  are  based  upon  the  same  chemical 
forces  which  are  involved  in  the  production  of  other  chemical 
compounds  makes  possible  a  rational  study  and,  it  may  confi- 
dently be  expected,  a  rational  understanding  of  such  compounds. 

Before  speaking  of  contact  reactions  farther,  it  may  be  well 
to  speak  of  a  factor  which,  to  the  writer  at  any  rate,  does  not 
appear  as  yet  to  have  been  accounted  for  in  a  satisfactory  man- 
ner. This  factor  is  the  part  which  is  played  by  some  chemical 
combining  force  in  various  phenomena.  As  stated  previously, 
Langmuir  defined  primary  valence  and  secondary  valence  in  a 
way  which  did  not  indicate  any  essential  difference  between  the 
two,  but  in  the  discussions  and  applications  he  used  them  to  rep- 
resent different  phenomena  which  in  many  cases  showed  definitely 
different  behaviors.  He  apparently  considered  that  the  transfer 
of  an  electron  to  form  a  definite  electron  grouping  or  configura- 
tion in  an  atom  or  molecule  involved  a  primary  valence,  while 
stray  fields  of  force  due  to  electrons  forming  the  outer  shells  of 
atoms  cause  secondary  valence  effects.  In  this  connection,  it 
may  be  mentioned  that  electrical  attraction  has  been  assumed 
at  various  times  to  play  a  part  in  the  formation  of  chemical 
compounds,  or  in  some  of  the  phenomena  observed  in  the  forma- 
tion or  reactions  of  such  compounds.  Thus,  A.  A.  Noyes  *  in 
connection  with  the  anomalies  observed  in  solutions  of  highly 
ionized  electrolytes  spoke  of  electrical  molecules  and  chemical 
molecules,  the  existence  of  the  former  being  due  to  electrical 
attraction  between  electrically  charged  atoms;  J.  M.  Nelson  and 

*A.  A.  Noyes,  Jour.  Amer.  Ghem.  Soc.  30,  351   (1908). 


150  '  CATALYTIC  ACTION 

the  writer  x  indicated  the  possibility  of  considering  J.  Thiele's 
partial  valences  on  the  basis  of  the  electron  conception  of  val- 
ence as  due  to  the  electrical  charges  on  atoms  resulting  from 
their  unions  with  other  atoms;  E.  C.  C.  Baly 2  developed  a 
theory  of  the  mechanism  of  chemical  reactions  on  the  basis  of 
condensed  force  fields  of  electromagnetic  type  surrounding  chem- 
ical molecules ;  and  a  number  of  others  might  be  quoted  who  sug- 
gested similar  views.  Langmuir  considered  such  a  force  field  in- 
volved in  the  phenomena  which  he  denoted  as  due  to  secondary 
valences,  while  Harkins  and  King  3  developed  a  similar  hypothe- 
sis and  applied  it  to  the  distribution  of  a  solute  between  various 
phases  as  conditioned  by  the  intermolecular  electromagnetic 
fields. 

These  phenomena  of  chemical  combination  form  the  uncer- 
tain note  in  the  explanation  of  certain  adsorption  phenomena, 
such  as  the  adsorption  of  some  of  the  so-called  permanent  gases 
by  solids,  while  at  the  same  time  many  other  cases  of  adsorption 
were  found  to  be  due  to  "primary"  or  ordinary  valence  forces. 
It  must  be  emphasized,  however,  that  such  surface  phenomena, 
whether  explained  as  due  to  secondary  valence  forces,  or  to  the 
actions  of  electromagnetic  fields,  or  to  any  other  cause,  are  also 
well  known  among  chemical  compounds  and  reactions  aside  from 
those  classed  as  adsorption  and  have  been  explained  by  the  terms 
secondary  valence,  auxiliary  valence,  electrical  attractions  of 
various  kinds,  force  fields,  etc.  Without  attempting  to  account 
for  the  actions  of  these  chemical  forces  in  any  final  and  definite 
manner,  it  may  be  said  that  the  forces  acting  to  form  these 
adsorption  compounds  differ  in  no  way  from  the  forces  which 
have  been  observed  in  other  reactions.  Their  interpretation  is 
not  as  direct  at  the  present  time  as  the  interpretation  of  the 
chemical  unions  which  involve  definite  changes  in  the  positions 
of  electrons,  but  it  appears  probable  that  an  explanation  for  the 
former  will  be  found  in  the  configurations  or  in  changes  in  the 
configurations  of  the  electrons  in  the  adsorbed  molecules  or  atoms 
and  the  adsorbing  surface.  An  explanation  involving  such  views 
was  suggested  by  Harkins  and  King.  At  any  rate,  the  state- 

1K.  G.  Falk  and  J.  M.  Nelson,  Jour.  Amer.  Ghem.  Soc.  32,  1650  (1910). 

2Cf.  Jour.  Amer.  Ghem.  Soc.  37,  979  (1915). 

1  W.  D.  Harkins  and  H.  H.  King,  Jour.  Amer.  Chem.  Soc.  41,  970  (1919). 


CONTACT  CATALYSIS  151 

ment  which  has  been  made  repeatedly  is  not  modified  by  these 
considerations,  that  the  forces  acting  in  adsorption  phenomena 
are  identical  with  those  acting  in  the  formation  and  reactions  of 
chemical  compounds  in  general. 

The  application  of  these  adsorption  views  to  chemical  reac- 
tions taking  place  in  heterogeneous  systems  is  obvious  in  prin- 
ciple. The  following  general  statement  of  Langmuir  1  may  be 
used  as  the  basis:  "In  a  heterogeneous  chemical  reaction,  the 
activity  of  a  surface  depends  in  general  upon  the  nature  of  the 
arrangement  of,  and  spacing  of  the  atoms  forming  the  surface 
layer."  It  is  of  interest  to  mention  in  passing,  the  earlier  at- 
tempts, from  which  were  developed  these  later  views,  to  formu- 
late a  general  theory  of  heterogeneous  or  contact  reactions, 
Noyes  and  Whitney  2  studied  the  solution  of  a  solid  in  a  liquid. 
The  rate  was  found  to  depend  upon  the  rate  of  diffusion  of  the 
saturated  solution  into  the  rest  of  the  liquid,  the  reaction  be- 
tween solid  and  liquid  (dissolving  of  solid  in  liquid)  being  ex- 
tremely rapid.  Nernst3  extended  this  conception  to  include  all 
reactions  occurring  in  heterogeneous  systems.  He  considered 
that  the  equilibrium  at  the  surface  of  two  phases  was  set  up 
very  rapidly,  practically  instantaneously,  in  comparison  with  the 
velocity  of  diffusion.  Since  the  equation  representing  the  ve- 
locity of  diffusion  was  similar  in  form  to  the  equation  of  a  mono- 
molecular  reaction,  whenever  the  latter  appears  to  hold  for  a 
reaction  taking  place  in  a  heterogeneous  system,  it  is  probable 
that  the  reaction  velocity  measured  is  that  of  a  rate  of  diffusion. 
This  simple  view  was  extended  by  Fink,4  who  showed  that  in 
the  mechanism  of  heterogeneous  reactions,  the  reaction  velocity 
was  limited  by  the  rate  of  diffusion  of  the  reacting  substances 
to  the  surface  where  the  reaction  was  taking  place  through  an 
adsorbed  film  of  variable  thickness  of  the  substances  taking 
part.  The  view  of  Langmuir  postulates  that  the  velocity  of 
heterogeneous  reactions  is  controlled  primarily  by  the  rate  at 
which  the  molecules  strike  against  that  portion  of  the  surface 
which  is  active.  It  was  recognized  that  physical  factors  such 
as  rates  of  diffusion  through  layers  of  gas  or  through  films  may 

1  I.  Langmuir,  Jour.   Amer.   Chem.   Soc.   37,  1142    (1915). 

2  A.  A.  Noyes  and  W.  K.  Whitney,  Z.  physik.  Chem.  23.  689   (1897) 

3  W.  Nernst,  Z.  physik.  Chem.  47,  52    (1904). 

*C.   G.    Fink,   Dissertation,   Leipsig,    1907;    M.    Bodenstein   and   C.    G     Fink 
Z.  phyaik.  Chem.  60,  1,  46   (1907). 


152  CATALYTIC  ACTION 

modify  the  conditions  or  limit  the  chemical  reaction  occurring, 
but  these  factors  were  considered  to  be  of  secondary  importance 
in  most  cases.  The  velocity  of  a  reaction  then  usually  depends 
on  the  fraction  of  the  surface  covered  by  adsorbed  atoms  or 
molecules,  which  in  turn  depends  on  the  rate  of  condensation  and 
on  the  rate  of  evaporation  (for  gases,  and  corresponding  rates 
for  liquids)  of  the  adsorbed  substance.  This  conception  was  de- 
veloped in  mathematical  form  to  cover  certain  special  cases  after 
making  some  simplifying  assumptions.  A  "law  of  surface  ac- 
tion" was  obtained  analogous  to  the  "law  of  mass  action."  The 
action  of  a  "poison"  was  shown  to  consist  of  the  formation  of  a 
very  stable  film  one  atom  or  molecule  deep. 

It  will  not  be  necessary  to  enter  farther  into  the  theory  and 
mathematical  deductions  given  by  Langmuir.  The  view  which 
he  presented  of  the  mechanism  of  contact  reactions  is  clear  and 
without  question  the  most  satisfactory  available  at  the  present 
time.  At  the  same  time,  the  conclusions  have  been  applied  quan- 
titatively to  comparatively  few  reactions.  On  the  other  hand, 
this  theory  offers  the  best  method  which  has  been  proposed  for 
classifying  contact  reactions  without  introducing  new  and  un- 
known factors  and  for  this  reason  has  been  outlined  here.  Also, 
it  brings  out  clearly  the  connection  between  the  mechanism  of 
contact  reactions  and  the  mechanism  of  chemical  reactions  in 
general  which  was  outlined  in  the  earlier  chapters.  Both  are 
based  upon  the  primary  formation  of  intermediate  or  addition 
compounds  of  the  reacting  substances.  If  one  of  the  substances 
in  a  contact  reaction  possesses  the  same  chemical  composition 
before  and  after  the  reaction,  then  the  reaction  would  be  classed 
as  contact  catalysis.  In  many  reactions  in  which  the  gaseous 
components  are  changed  chemically,  a.  substance  in  the  solid 
state  is  involved,  which  sometimes  acts  as  the  container,  but  is 
unchanged  in  composition  at  the  end  of  the  reaction.  Such  reac- 
tions are  included  in  contact  catalysis  and  form  probably  the 
principal  reactions  of  that  group.  The  general  formulations 
given  in  the  earlier  chapters  to  represent  the  mechanism  of  chemi- 
cal reactions  are  directly  applicable  to  contact  reactions. 

Because  of  the  importance  of  the  theoretical  views  of  Lang- 
muir, some  of  his  experimental  work  may  be  given  first.  His 


CONTACT  CATALYSIS  153 

earlier  work  on  chemical  reactions  at  low  pressures x  included  the 
"clean-up"  of  a  gas  by  a  heated  filament.  The  four  types  of 
reaction  studied  included  the  direct  attack  of  the  filament  by  the 
gas  (tungsten  and  oxygen  or  chlorine,  carbon  and  oxygen) ;  the 
action  of  the  gas  with  the  vapor  given  off  by  the  filament 
(tungsten  and  nitrogen  or  carbon  monoxide,  molybdenum  and 
nitrogen,  platinum  and  oxygen) ;  the  catalytic  action  of  the  fila- 
ment on  the  gas,  producing  a  chemical  change  in  the  gas  with- 
out any  permanent  change  in  the  filament  (dissociation  of  hy- 
drogen, chlorine,  and  oxygen  into  atoms  by  heated  tungsten, 
platinum,  or  palladium,  as  well  as  other  chemical  changes) ;  and 
complicated  reactions  in  which  electrical  discharges  ionized  the 
gas  and  brought  out  reactions  between  ionized  gas  and  filament 
(tungsten  and  nitrogen).  These  reactions  were  all  readily  ex- 
plained by  means  of  the  theory  of  chemical  combination  (with 
the  filament  or  other  body)  as  already  outlined.  A  more  detailed 
study  was  made  of  the  action  of  carbon  monoxide  and  oxygen 
and  of  hydrogen  and  oxygen  with  platinum.  The  results  were 
summarized  as  follows:  2  "No  adsorption  of  gases  could  be  ob- 
served even  at  — 183°,  until  the  platinum  had  been  "activated" 
by  heating  to  300°  in  a  mixture  of  hydrogen  and  oxygen  at  low 
pressure.  After  this  activation,  hydrogen  and  oxygen  and  car- 
bon monoxide  and  oxygen  reacted  together  readily  at  room  tem- 
perature in  contact  with  the  platinum.  The  platinum  was  then 
found  capable  of  adsorbing  oxygen,  carbon  monoxide  or  hydro- 
gen. The  maximum  quantities  of  oxygen  and  carbon  monoxide 
corresponded  to  monomolecular  layers.  The  oxygen  could  not 
be  driven  off  either  by  heat  or  by  pumping.  When  the  platinum 
was  in  contact  with  an  excess  of  oxygen  the  amount  of  oxygen 
adsorbed  increased  as  the  temperature  was  raised,  but  the  action 
was  irreversible.  Adsorbed  carbon  monoxide  could  not  be  re- 
moved at  room  temperature,  but  at  300°  part  of  it  could  be 
pumped  off.  When  oxygen  was  brought  in  contact  with  carbon 
monoxide  adsorbed  on  the  platinum  it  reacted  rapidly  to  form  V 
carbon  dioxide  which  at  room  temperature  showed  no  tendency 
to  be  adsorbed  on  the  platinum.  In  a  similar  way  carbon 
monoxide  brought  into  contact  with  adsorbed  oxygen  reacted 

*!.  Langmuir,  Jour.  Amer.  Chem.  Soc.  37,  1139  (1915). 
*  I.  Langmuir,  Jour.  Amer.  Chem.  Soc.  40,  1402  (1918). 


154  CATALYTIC  ACTION 

immediately.  These  cases  of  adsorption  are  clearly  due  to  chem- 
ical forces  of  the  primary  valence  type.  Further  work  needs  to 
be  done  to  determine  the  cause  of  the  activation  of  the  platinum." 
If  a  gas  forms  an  adsorbed  film  on  a  surface,  for  example 
platinum,  and  if  the  gas  evaporates  slowly,  a  platinum  surface 
would  no  longer  be  present.  The  catalytic  activity  of  the  plati- 
num would  be  lost,  or  the  catalyst  is  said  to  have  been  poisoned. 
Arsenic,  antimony  and  phosphorus  have  this  effect;  evidently 
they  are  combined  with  the  platinum  and  do  not  evaporate  at 
an  appreciable  rate.  Cyanogen  and  carbon  monoxide  have  simi- 
lar, though  more  transient  effects  on  platinum;  they  "poison" 
the  platinum  only  as  long  as  some  of  the  substance  is  present 
in  the  gaseous  phase.  If  a  reaction  takes  place  in  the  presence 
of  a  gas  which  has  a  poisoning  action  on  the  catalyst,  the  velocity 
will  depend  on  the  fraction  of  the  surface  not  covered  by  the 
molecules  of  the  poisoning  gas. 

"When  gas  molecules  condense  on  a  solid  surface  in  such  a  way  that 
they  are  held  on  the  surface  by  primary  valence  forces,  involving  a  re- 
arrangement of  their  electrons,  their  chemical  properties  become  completely 
modified.  It  is  not  surprising,  therefore,  that  in  some  cases  such  adsorbed 
films  should  be  extremely  reactive,  while  in  other  cases  they  may  be  very 
inert  to  outside  influences.  Thus  oxygen  adsorbed  on  platinum  reacts 
readily  with  hydrogen  or  carbon  monoxide,  while  oxygen  on  tungsten,  or 
carbon  monoxide  on  platinum,  show  very  little  tendency  to  react  with 
gases  brought  into  contact  with  their  surfaces.  The  specific  nature  of  the 
behavior  of  these  various  films  is  quite  consistent  with  the  theory  that  the 
adsorption  depends  on  typical  chemical  action.  In  many  cases,  especially 
where  we  deal  with  adsorption  of  large  molecules,  the  orientation  of  the 
molecules  on  the  surface  is  a  factor  of  vital  importance  in  determining  the 
activity  of  the  surface  towards  reacting  gases."1 

Langmuir  also  considered  the  effect  of  treatment  on  a  sur- 
face. For  example,  the  occurrence  of  a  reaction,  or  rapid  heat- 
ing and  cooling  as  by  an  alternating  current,  roughens  the  sur- 
face of  metals  such  as  platinum,  etc.  In  this  way  the  surface 
area  becomes  larger  and  a  greater  number  of  favorable  locations 
for  action  is  produced.  At  the  same  time  the  significant  fact  must 
be  noted  that  the  surface  may  become  activated  for  one  reaction 
and  not  for  another.  Thus,  a  plane  surface  of  platinum  acti- 
vated (lowering  of  temperature  at  which  reaction  began) 
toward  the  hydrogen- oxygen  reaction  by  a  single  treatment  at 

»I.  Langmuir,  Chem.  News  123,  225   (1921). 


CONTACT  CATALYSIS  155 

low  pressures  with  a  hydrogen-oxygen  mixture  was  not  activated 
toward  the  carbon  monoxide-oxygen  reaction.1 

These  results  showed  clearly  the  chemical  nature  of  the  forces 
involved  in  these  adsorptions  and  contact  actions.  Even  with 
these  comparatively  simple  reactions,  the  mechanism  of  the 
changes  may  be  quite  complex  and  present  features  which  have 
not  been  satisfactorily  elucidated.  Some  recent  work  on  the 
hydrogenation  of  aromatic  compounds  with  the  aid  of  platinum 
or  palladium 2  also  indicated  that  the  "activation"  of  these 
metals  might  involve  a  more  complex  change  than  is  apparent 
at  first  sight.  Catalytic  hydrogenation  was  found  to  depend  on 
the  presence  of  oxygen;  the  intermediate  products  were  not 
hydrides  but  contained  both  oxygen  and  hydrogen.  Completely 
deoxygenated  palladium  sponge  or  colloidal  palladium  was  found 
to  be  incapable  of  hydrogenating  even  such  compounds  as 
diolefins. 

One  of  the  most  important  features  of  chemical  compounds 
is  their  individuality.  Each  atom  or  molecule  will  act  in  a 
more  or  less  specific  way  with  various  reagents.  The  reactions 
which  are  used  in  qualitative  and  quantitative  chemical  analyses 
take  advantage  of  these  differences.  Certain  reactions  which 
are  grouped  under  contact  catalysis  have  been  used  in  quantita- 
tive analysis.  The  reactions  to  which  reference  is  made  espe- 
cially are  the  so-called  "preferential  combustion"  reactions,  in 
which  gaseous  mixtures  may  be  analyzed  by  fractional  or  par- 
tial combustion  (oxidation)  of  the  constituents.  The  separate 
estimations  of  hydrogen  and  methane  may  be  quoted  as  illus- 
trating the  possibilities  of  such  reactions.3  If  a  mixture  of 
hydrogen  and  methane  is  passed  over  cupric  oxide  at  250°,  the 
hydrogen  will  be  oxidized  completely  while  the  methane  will  not 
be  attacked.  The  use  of  cupric  oxide  is  stated  to  be  preferable 
to  that  of  palladium  or  palladinized  asbestos  because  the  addition 
of  air  or  of  oxygen  is  obviated.  Also,  hydrogen  is  oxidized  be- 
fore methane  at  comparatively  low  temperatures  in  contact  with 

1  For   further    evidence    relative    to    these   phenomena    cf.    P.    Woog,    Comnt. 
rend,  m,  387  (1921)  ;  N.  K.  Adam,  Proc.  Roy.  Soc.  London  (A)  99,  336   (1921)  ; 
H.  S.  Taylor  and  R.  M.  Burns,  Jour.  Amer.  Chem.  Soc.  43,  1273   (1921)  ;  H.  S. 
Taylor  and  H.  A.  Neville,  Jour.  Amer.  Chem.  Soc.  43,  2055   (1921). 

2  R.  Willstatter  and  E.  Waldschmidt-Leitz,  Ber.  54B,  113   (1921).     Cf.  how- 
ever C.  Kelber,  Ber.  54B,  1701   (1921). 

3Cf.  W.  D.  Bancroft,  J.  Physic.  Chem.  21,  044  (1917),  for  a  summary  of 
reactions  involving  fractional  combustion  of  gases. 


156  CATALYTIC  ACTION 

platinum,  or  at  higher  temperatures  if  the  mixture  of  gases  is 
passed  through  a  platinum  tube.  Methane  is  found  to  be  oxi- 
dized in  preference  to  hydrogen  at  moderate  temperature  in 
borosilicate  glass  bulbs  and  if  the  mixture  of  gases  is  fired  by 
an  electric  spark. 

Similar  preferential  combustion  reactions  have  been  studied 
for  mixtures  of  gases  containing  hydrogen,  carbon  monoxide, 
various  hydrocarbons,  etc.,  involving  the  use  of  a  number  of 
metals  or  oxides.  A  satisfactory  review  of  such  reactions  to- 
gether with  references  to  the  original  papers  was  given  recently 
(1919)  by  E.  K.  Rideal  and  H.  S.  Taylor  in  their  book  on 
"Catalysis  in  Theory  and  Practice."  x 

The  question  of  hydrogenation  reactions  in  general  belongs  to 
the  reactions  under  discussion.  The  reactions  described  and 
summarized  by  P.  Sabatier,  who  did  a  great  deal  of  the  work 
himself,  in  his  book 2  on  "La  Catalyse  en  Chimie  Organique" 
included  in  the  main,  contact  catalytic  reactions.  He  assumed,  as 
the  most  probable  explanation  for  the  mechanism  of  such  reac- 
tions, the  formation  of  an  intermediate  compound  of  catalyst 
with  the  reacting  substances.  A  similar,  but  perhaps  more  defi- 
nite point  of  view  was  developed  by  E.  F.  Armstrong  and  T.  P. 
Hilditch  in  a  series  of  studies  on  "Catalytic  Actions  at  Solid 
Surfaces"  3  in  which  various  metals  were  used  in  hydrogenation 
reactions.  Some  of  their  statements  are  pertinent  to  the  problem 
under  discussion.  For  example,  they  adopted  the  theory  of  the 
formation  of  unstable  intermediate  compounds  in  such  reactions, 
and  considered  the  physical  view  of  adsorption  (as  dependent 
upon  chemical  forces)  to  be  identical  with  the  chemical  hypoth- 
esis of  unstable  intermediate  compounds ;  4  that  "the  primary 
action  of  the  catalyst  in  all  these  cases  is  to  effect  an  association 
with  the  carbon  compound,  the  resulting  unstable  complex  then 
being  resolved  into  other  compounds"; 5  that  the  activity  of  the 
catalyst  (nickel  in  the  special  case  under  investigation)  de- 

1Pp.  439-446.  The  recent  studies  by  C.  Conover  and  H.  D.  Gibbs  (J  Ind 
Eng.  Chem.  Ik,  120  (1922)  on  the  oxidation  of  naphthalene  to  phthalic  anhydride 
by  air  in  the  presence  of  various  oxides  at  higher  temperatures  may  also  be 
mentioned.  Vanadium  pentoxide  was  found  to  give  the  best  results ;  molyb- 
denum trioxide  gave  fairly  good  results ;  while  a  large  number  of  other  oxides 
were  found  to  be  poor  or  worthless  for  the  purpose. 

2  Published  in  1913  ;  second  edition,  1920. 

•  Published  in  Proc.  Roy.  Soc.  London,,  1919-1921. 

*  Proc.  Roy.  Soc.  London  (A)  98,  27   (1920). 
5  lUd.  97,  259   (1920). 


CONTACT  CATALYSIS  157 

pended  upon  a  suitable  surface  and  the  specific  nature  of  the 
catalyst  in  being  able  to  form  appropriate  intermediate  com- 
pounds with  the  reacting  substances;  x  etc. 

A  number  of  additional  workers  might  be  quoted  to  the  same 
effect  2  relative  to  reactions  similar  to  those  described,  but  this 
would  not  add  to  the  general  point  of  view  outlined.  The  mech- 
anisms of  the  reactions  can  evidently  be  accounted  for  readily 
according  to  the  principles  developed  even  if  exact  quantitative 
data  are  not  always  available.  It  will  perhaps  be  of  more  in- 
terest to  take  up  reactions  of  somewhat  different  nature  or  in- 
volving chemical  changes  different  from  those  heretofore  con- 
sidered. 

The  decomposition  of  ethyl  alcohol  may  take  place  accord- 
ing to  the  following  reactions  as  indicated  in  Chapter  III: 


H  OH 
5 


~1    =  ^2^  +  C2H4  (a)  /j  x 

J   =H2  +  CH3CHO  (b) 


A  study  of  the  products  obtained  when  alcohol  was  heated  in  the 
presence  of  a  number  of  oxides  and  of  metals  gave  the  following 
results:3  In  the  presence  of  thorium  oxide  (all  the  oxides  were 
prepared  at  temperatures  below  350°)  at  340-350°,  the  reaction 
followed  equation  (a)  exclusively;  with  aluminium  oxide  or 
tungstic  oxide  (blue),  the  reaction  followed  (a)  almost  entirely, 
very  little  hydrogen  (1.5%  by  volume  of  gas)  being  formed; 
with  chromium  oxide,  silica,  or  titanium  oxide,  the  reaction 
took  place  according  to  both  equations  with  (a)  predominating; 
with  beryllium  oxide  or  zirconium  oxide  about  one-half  of  the 
decomposition  followed  each  equation;  with  uranium  oxide, 
molybdenum  oxide  (blue),  ferric  oxide,  vanadous  oxide,  or  zinc 
oxide,  considerably  more  of  the  products  of  equation  (b)  than 
of  equation  (a)  were  formed;  while  with  manganese  dioxide, 
stannous  oxide,  cadmium  oxide,  manganous-manganic  oxide 
(Mn304),  magnesium  oxide,  or  finely  divided  nickel  or  copper, 
the  reaction  took  place  entirely  according  to  equation  (b)  .  The 
reaction  according  to  equation  (b)  was  complicated  in  some 
cases  by  the  decomposition  of  the  acetaldehyde  into  methane 

1  Proc.  Roy.  Soc.  London  (A)   99,  490   (1921). 

2  Cf.    H.   Euler  and   A.    Hj.   Hedelius,   Artoic.   Kemi,  Mineral  Geol.    7,  No.   31 
(1920). 

3P,  Sabaticr  and  A.  Mailhe,  Ann,  chim.  phys.   (8)   20,  289  (1910). 


158  CATALYTIC  ACTION 

and  carbon  monoxide.  The  efficiency  of  these  substances  as 
catalysts  varied  greatly,  the  gases  evolved  per  minute  under  the 
experimental  conditions  used  ranging  from  traces  with  magne- 
sium oxide  and  0.9  cubic  centimeter  with  silica  up  to  57  cubic 
centimeters  with  tungstic  oxide  and  110  cubic  centimeters  with 
copper.  A  study  1  of  the  addition  of  water  vapor  and  of  hydro- 
gen to  the  reaction  mixtures  showed  some  definite  influences  on 
the  displacement  of  the  equilibria  with  the  different  catalysts, 
but  some  more  recent  results  indicate  that  further  studies  2  of 
the  mechanism  of  the  reactions,  perhaps  similar  to  those  carried 
on  by  Langmuir  with  different  reactions,  are  necessary  in  order 
to  account  for  the  observed  effects. 

Similar  decompositions  have  been  described  with  other  alco- 
hols. Thus,  isobutyl  alcohol  at  300°  formed  isobutyl  aldehyde 
and  hydrogen  in  the  presence  of  copper,  isobutylene  and  water 
in  the  presence  of  aluminium  oxide,  and  both  the  aldehyde  and 
isobutylene  in  the  presence  of  uranium  oxide.3  The  results  with 
allyl  alcohol  have  also  been  published.4 

Formic  acid  may  decompose  according  to  the  following 
equations:  5 


r 


=H20  +  CO  (a) 

'°H      =H2  +  C02  (b) 


Reaction  (a)  takes  place  on  warming  formic  acid  with  sulfuric 
or  other  mineral  acids,  reaction  (b)  by  heating  formic  acid 
with  platinum  or  with  finely  divided  rhodium,  ruthenium,  or 
iridium,  or  with  an  excess  of  alkali.  A  recent  study  of  the  de- 
composition of  salts  of  formic  acid  by  heat,  or  by  passing  formic 
acid  vapor  over  heated  metallic  oxides  where  the  first  products 
formed  were  the  metal  formates  which  then  decomposed,  gave 
some  interesting  results.6  The  reaction  primarily  was  consid- 
ered to  be  as  follows: 

(CH02)2  Me"  =  MeC03  +  ECHO,  (3) 

JC.  J.  Engelder,  J.  Physic.  Ohem.  21,  676  (1917). 

2  W.  D.  Bancroft,  Address  on  "Contact  Catalysis,"  American  Electrochemi- 
cal Society,  1920. 

8  P.  Sabatier,  "La  Catalyse  en  Chimic  Organique,"  German  translation,  1914, 
p.  240. 

*P.  Sabatier  and  B.  Kubota,  Compt.  rend.  113,  17,  212   (1921). 

6  Cf.  Chapter  III,  p.  41. 

«K,  A.  Hpfmann  and  H.  Schibsted,  Ber.  51,  1398  (1918). 


CONTACT  CATALYSIS  159 

in  which  the  nature  of  the  contact  substance  or  metal  oxide, 
formate,  or  carbonate,  as  well  as  the  manner  of  heating,  water 
content,  and  other  factors,  determined  the  various  further 
changes  which  took  place.  With  zinc  formate,  a  yield  of  25 
per  cent  of  formaldehyde  could  be  obtained;  with  lithium  formate 
(perhaps  partly  because  of  the  high  temperature  required  to 
cause  decomposition)  very  little  formaldehyde  was  found,  the 
main  products  being  acetone,  methyl  alcohol,  furfurol,  pyruvic 
acid,  and  charcoal.  Lead  formate  gave  formaldehyde  and  much 
methyl  alcohol;  cobalt  formate,  only  traces  of  methyl  alcohol; 
aluminium  formate,  no  formaldehyde  or  methyl  alcohol;  stan- 
nous  formate,  a  good  yield  of  formaldehyde  and  almost  no  methyl 
alcohol;  etc.  The  results  were  given  for  a  number  of  addi- 
tional salts  and  the  effects  of  varying  the  conditions,  such  as 
temperature,  addition  of  various  substances  to  the  reaction  mix- 
ture, etc.,  carefully  studied.  The  apparent  irregularity  of  the 
results  made  it  difficult  to  draw  any  general  conclusions  relative 
to  the  reasons  for  the  different  actions  of  the  different  salts. 

The  decomposition  of  ethyl  acetate,  when  heated  with  dif- 
ferent solid  catalysts,  may  proceed  according  to  the  following 
equations: 

F  -|  =  CH3C02H  +  C2H4  (a) 

nCH3C02C2H5     =  CH3CH2CH3  +  C02  (b) 

L  J  -  CH3COCH3  +  C02  +  C2H5OH  +  C2H4  (c) 

(4) 

Reaction  (a)  takes  place  in  the  presence  of  titanium  oxide;  re- 
action (b)  with  finely  divided  nickel;  and  reaction  (c),  with 
thorium  oxide.1  According  to  Langmuir's  view,  the  —  CO.O  - 
group  is  probably  attached  to  the  catalyst  surface  by  means  of 
ordinary  valences,  and,  "depending  upon  the  different  manners 
in  which  interaction  between  atoms  and  evaporation  may  occur, 
the  resulting  products  differ."  2 

The  results  obtained  with  some  simple  compounds  of  nitro- 
gen may  be  quoted.3  Hydroxylamine  in  a  hot  alkaline  solution 

1  These  results  'were  obtained  by  Sabatier.  The  recently  published  work  of 
H.  Adkins  and  A.  C.  Krause  (Jour.  Amer.  Chem.  Soc.  44,  385  (1922))  indicates 
that  these  conclusions  require  revision. 

•I.  Langmuir,  Chem.  News  123,  237   (1921). 

3  S.  Tanatar,  Z.  physift.  Chem.  J^O,  475  (1902), 


160  CATALYTIC  ACTION 

was  found  to  decompose  mainly  according  to  the  following  equa- 
tion: 

3NH30  ='NH3  +  N2  +  3H20  (5) 

In  the  presence  of  platinum  black,  however,  the  reaction  pro- 
ceeded principally  as  follows: 

4NH30  =  2NH3  +  N20  +  3H20  (6) 

The  actions  of  the  alkali  in  the  reaction  shown  in  equation  (5) 
and  of  the  platinum  black  in  the  reaction  of  equation  (6)  are 
not  given  in  these  equations.  Only  the  latter  would  represent  a 
case  of  contact  catalysis. 

It  is  difficult  at  the  present  time  to  say  more  relative  to  these 
reactions.  The  nature  of  the  catalysts  and  the  conditions  under 
which  the  changes  are  allowed  to  take  place  evidently  play  a 
most  important  part  in  determining  the  character  of  the  products. 
It  is  especially  clear,  in  considering  such  reactions  as  the  above, 
that  the  catalyst  plays  an  active  part  in  these  chemical  reac- 
tions, and  that  an  understanding  of  the  mechanism  of  the  changes 
involves  necessarily  the  participation  of  the  catalyst  in  the  reac- 
tion and  the  representation  of  this  participation  in  the  chemical 
equation  which  is  supposed  to  indicate  the  changes  taking  place, 
as  well  as  in  all  other  equations  or  expressions  which  may  be 
intended  to  be  descriptive  of  the  reactions  in  question. 

It  would  be  possible  to  extend  the  list  of  contact  catalytic 
reactions  and  to  discuss  the  possible  mechanism  of  the  changes 
on  the  basis  of  the  data  of  various  kinds  which  are  available. 
The  reactions  which  have  been  given  are  only  a  few  of  those 
which  might  be  given,  and  perhaps  not  the  most  important,  nor, 
from  some  points  of  view,  the  most  interesting.  Thus,  the  Haber 
process  of  ammonia  synthesis,  the  contact  sulfuric  acid  process, 
the  recent  striking  developments  of  the  application  of  silica  gel 
to  a  variety  of  processes  (the  latter,  however,  being  essentially 
condensation  phenomena),  etc.,  would  permit  of  almost  unlim- 
ited discussion.  These  reactions,  and  especially  the  first  two, 
have  been  taken  up  in  detail  in  various  connections  in  the 
chemical  literature.  •  As  far  as  the  writer  is  aware,  no  new 
principles  have  been  developed  in  the  study  of  these  reactions, 


CONTACT  CATALYSIS  161 

differing  from  those  outlined  here.  An  enumeration  of  additional 
reactions,  therefore,  which  might  be  included  under  Contact 
Catalysis  will  be  dispensed  with  in  this  connection. 

The  term  "promoter"  action  in  catalysis  has  been  used  to 
indicate  the  influence  of  various  substances  in  increasing  the  ac- 
tions of  catalysts.  In  a  recent  review  entitled  "Promoter  Action 
in  Catalysis,"  Pease  and  Taylor *  made  a  distinction  between 
(1)  Activation  of  a  catalyst  by  a  substance  relatively  inert  cata- 
lytically  or  by  a  small  quantity  of  relatively  inactive  substance, 
and  (2)  Co-activation  of  two  or  more  catalytically  active  sub- 
stances each  by  the  others.  They  gave  neutral  salt  actions  in 
catalyses  by  hydrogen  ions  in  aqueous  solutions,  etc.,  as  examples 
of  the  first  group  of  actions,  and  the  actions  of  iron  and  of 
molybdenum  and  of  a  mixture  of  the  two  on  the  ammonia  syn- 
thesis from  nitrogen  and  hydrogen  as  an  example  of  the  second. 
A  number  of  reactions  might  be  quoted  to  illustrate  this  promoter 
action.  For  example,  the  oxidation  of  carbon  monoxide  to  carbon 
dioxide  by  the  oxygen  of  the  air  at  ordinary  temperatures  by  a 
mixture  of  three  or  four  oxides  is  a  case  in  point;2  the  increased 
hydrogenation  of  fats  by  the  addition  of  tellurium  to  nickel; 
etc.8 

In  view  of  the  striking  character  of  some  of  these  promoter 
actions,  it  is  surprising  that  no  theory,  based  upon  experimental 
evidence,  has  so  far  been  suggested  which  accounts  for  these 
actions  in  a  satisfactory  way. 

The  enzyme  actions  which  were  described  in  Chapter  VI  be- 
long at  least  in  part  to  the  group  of  contact  catalytic  reactions. 
All  enzymes,  as  far  as  known,  are  associated  with,  or  form  part 
of,  matter  in  the  colloid  state.  Even  when  the  solutions  con- 
taining enzymes  appear  to  be  clear  to  the  eye,  as  with  some 
sucrase  and  esterase  preparations,  the  enzyme  substance  is  not 
able  to  pass  through  a  suitable  semi-permeable  membrane  such 
as  collodion  or  parchment,  while,  to  go  to  the  other  extreme,  a 
number  of  enzymes  are  practically  insoluble  and  can  be  sepa- 
rated from  their  solutions  or  mixtures  by  means  of  simple  fil- 

1E.  N.  Pease  and  H.  S.  Taylor,  J.  Physic.  Chem.  24,  241  (1920). 

2  A.  B.  Lamb,  W.  C.  Bray,  and  J.  C.  W.  Frazer,  J.  Ind.  Eng.  Chem.  n,  213 
(1920)  ;  T.  H.  Rogers,  C.  S.  Piggot,  W.  H.  Bahlke,  and  J.  M.  Jennings,  Jour. 
Amer.  Chem.  Soc.  43>  1973  (1921)  ;  D.  Merrill  and  S.  C.  Scalione,  Jour.  Amer. 
Chem.  Soc.  43,  1982  (1921). 

-«.,ftSCf-  ?A  *£  Rideal  and  H-  s-  Taylor,  "Catalysis  in  Theory  and  Practice," 
1919,  pp.  29-32,  for  additional  examples. 


162  CATALYTIC  ACTION 

tration  through  ordinary  filter  paper.  These  also  show  colloidal 
properties.  The  question  which  may  be  raised  relates  to  the  ex- 
tent to  which  the  views  on  contact  reactions  apply  to  enzyme 
actions.  The  summary  of  the  results  of  the  study  of  enzyme 
actions  presented  in  Chapter  VI  as  well  as  the  relations  pointed 
out  in  other  connections  x  falls  in  with  the  chemical  relations  de- 
veloped by  Langmuir  although  the  question  of  the  nature  of  the 
surface  actions  is  not  directly  applicable,  or  at  least  not  well 
enough  defined  at  the  present  time.  The  first  step  in  the  action 
between  enzyme  and  substrate  is  a  chemical  combination  be- 
tween the  two  to  form  the  so-called  addition  compound.  Because 
of  the  colloidal  nature  of  the  enzyme  preparations  as  well  as  the 
colloidal  nature  of  a  number  of  the  substrates  which  are  used 
in  enzyme  studies,  this  action  frequently  has  been  considered  to 
be  an  "adsorption"  phenomenon  with  the  chemical  forces  play- 
ing at  most  a  secondary  part.2  The  view  of  Langmuir  relative 
to  the  part  played  by  chemical  forces  in  forming  adsorption  com- 
pounds brings  the  two  explanations  together.  The  limiting 
amount  of  substrate  with  which  a  definite  (small)  amount  of 
enzyme  can  react  in  a  unit  of  time  is  also  suggestive  of  the  extent 
of  surface  which  can  be  covered  by  the  adsorbed  substance. 
Both  are  fundamentally  perhaps  only  different  forms  of  the  law 
of  definite  proportions  in  chemical  combinations.  Further,  the 
combination  of  the  products  of  an  enzyme  action  with  the  enzyme 
prevents  the  further  action  of  the  enzyme  on  fresh  substrate. 
This,  also,  is  comparable  to  the  "poisoning"  of  a  surface  by  the 
adsorption  by  chemical  forces,  of  a  substance  which,  by  covering 
the  surface  completely  or  even  to  a  large  extent,  prevents  differ- 
ent substances  from  reaching  the  surface  and  reacting  with  the 
catalyst  substance  and  with  each  other.  Enzyme  actions  and 
contact  actions  on  solid  surfaces  can  be  treated  from  the  chemical 
point  of  view  as  chemical  reactions  which  are  apparently  some- 
what more  complex  than  those  ordinarily  dealt  with,  as,  for  ex- 
ample, reactions  in  aqueous  solutions.  The  apparent  complexity 
is  due  in  part  to  the  fact  that  the  former  reactions  involve  sub- 
stances present  initially  in  different  phases  and  also  possibly  be- 

1  Cf.   "The  Chemistry  of  Enzyme   Actions." 

2Cf.  for  example  Chapter  VII  on  "The  Mode  of  Action  of  Enzymes"  in  "The 
Nature  of  Enzyme  Action,"  by  W.  M.  Bayliss ;  Fourth  Edition ;  1919. 


CONTACT  CATALYSIS  163 

cause  these  reactions  take  place  in  stages  whose  velocities  fre- 
quently are  of  the  same  order  of  magnitude.  It  might  be  ex- 
pected, because  of  their  colloid  nature  and  the  reactions  taking 
place  in  heterogeneous  systems,  that  the  actions  of  enzymes 
would  be  treated  theoretically  in  the  same  way  that  contact  re- 
actions have  been  treated.  As  a  matter  of  fact,  in  his  treatment 
of  contact  phenomena,  Langmuir  l  presented  a  brief  outline  in 
which  he  applied  his  views  to  enzyme  actions,  but  made  some- 
additional  assumptions  with  regard  to  the  mechanism  of  the 
actions.  J.  M.  Nelson  and  W.  C.  Vosburgh  2  pointed  out  the 
limitations  of  these  assumptions,  at  least  with  respect  to  the 
action  of  sucrase,  but  in  all  probability  applying  also  to  other 
enzyme  actions.  In  fact,  it  is  doubtful  whether  such  a  general 
treatment  as  that  indicated  by  Langmuir  could  state  more  than 
possible  empirical  rules  of  action,  unless  the  chemical  natures  of 
the  reacting  substances  are  introduced.  This  last  unfortunately 
appears  to  be  almost  impossible  at  the  present  time  in  view  of  the 
unknown  chemical  configurations  of  enzyme  molecules  and  the 
uncharacterized  physical  structures  of  the  surfaces  of  enzyme 
preparations. 

Reactions  which  occur  in  heterogeneous  systems  are  of  in- 
terest in  connection  with  changes  which  occur  in  living  matter  as 
indicated  in  the  preceding  chapter.  The  question  of  the  phe- 
nomena which  occur  in  or  at  the  surfaces  of  cells,  in  which  the 
cell  walls  or  membranes  play  a  part,  appears  to  be  related  to 
the  problems  which  have  been  taken  up  in  this  chapter  relative 
to  contact  actions.  Only  a  few  of  the  possibilities  can  be  men- 
tioned here. 

The  question  of  osmosis  and  the  existence  of  so-called  semi- 
permeable  membranes  and  their  actions  is  involved  in  these  con- 
siderations. This  is  not  the  place  to  enter  into  a  detailed  discus- 
sion of  these  phenomena  although  they  are  of  fundamental  im- 
portance for  an  understanding  of  the  relations  as  far  as  they 
have  been  developed.3  Only  a  few  of  the  more  salient  points 
can  be  taken  up.  In  the  first  place,  the  process  by  which  such 

1 1.  Langmuir,  Jour.  Amer.  Chem.  Soc.  38,  2291   (1916). 

2J.  M.  Nelson  and  W.  C.  Vosburgh,  Jour.  Amer.  Chem.  Soc.  39,  805   (1917). 
3  Cf.  especially  the  studies  of  J.  Loeb   published  in  J.  Gen.  Physiol.   1919- 
1921. 


164  CATALYTIC  ACTION 

a  membrane  acts  has  been  studied  extensively.    In  his  mono- 
graph on  "Osmotic  Pressure,"  Findlay  stated:  x 

"The  explanation  of  semi-permeability  of  membranes  which  is  probably 
the  most  widely  accepted  at  the  present  day,  is  that  of  selective  or  prefer- 
ential solubility.  The  membrane  is  permeable  to  those  substances  which 
dissolve  in  it,  and  is  impermeable  to  those  substances  which  are  insoluble 
in  it." 

This  view  shows  that  some  form  of  combination  is  involved 
between  a  substance  and  a  membrane  when  that  substance  is 
capable  of  passing  through  the  membrane.  Further,  it  is  pos- 
sible that  the  actions  may  be  of  the  nature  of  adsorption  as 
defined  by  Langmuir  instead  of  solution  as  stated.  The  struc- 
tures of  membranes,  being  of  more  or  less  porous  character,  would 
make  it  difficult  to  decide,  without  careful  study,  which  phe- 
nomenon was  taking  place.  In  fact,  the  whole  problem  appears 
to  rest  upon  the  specific  properties  of  membranes.  A  thorough 
review,  with  references  to  the  original  literature,  was  given  by 
W.  M.  Bayliss  in  his  "Principles  of  General  Physiology,"  2  Chap- 
ter V,  on  "The  Permeability  of  Membranes  and  the  Properties 
of  the  Surface  of  Cells,"  to  which  reference  may  be  made  for 
more  detailed  information. 

The  cell  membrane,  if  formed  of  different  materials  from  the 
cell  contents  (possibly  cellulose  in  the  case  of  plant  cells)  or  the 
cell  surface  which  may  be  formed  from  the  protoplasm  of  the 
cell,  which,  because  of  the  surface  energy,  or  of  force  fields  at 
the  surface  due  to  the  arrangements  of  atoms  at  the  surface  re- 
sulting for  one  thing  in  a  change  of  surface  energy,  as  postu- 
lated by  Langmuir  and  by  Harkins,  would  show  the  phenomena 
of  contact  actions  toward  foreign  substances,  which  were  de- 
scribed in  the  earlier  parts  of  this  chapter.  A  substance  would 
combine  with  the  membrane  or  surface  because  of  chemical 
forces.  The  reactions  would  be  specific.  The  further  behavior 
of  the  combined  substance  might  involve  reaction  with  the  proto- 
plasm in  the  interior  of  the  cell,  or  reaction  with  the  substance 
of  the  cell  surface,  or  possibly  other  changes.  In  view  of  the 
lack  of  definite  knowledge  of  these  phenomena  at  the  present 
time,  it  would  be  idle  to  enter  farther  into  these  questions.  The 

1  Monographs    on    Inorganic    and    Physical    Chemistry ;    "Osmotic    Pressure" 
by  A.  Findlay ;  published  by  Longmans,  Green  and  Co.,  1913.     P.  68. 

2  Published  by  Longmans,  Green  &  Co.,  1915. 


CONTACT  CATALYSIS  165 

reactions  are  fundamentally  contact  reactions.  If  the  cell  mem- 
brane or  surface  is  unchanged  after  the  reaction,  the  change 
would  be  classed  as  contact  catalysis. 

These  considerations  do  not  touch  upon  the  actions  of  the 
cell  enzymes  upon  the  material  supplied  to  and  taken  up  by  the 
cells.  There  would  appear  to  be  a  connection  or  interdependence, 
necessarily,  between  the  "permeability"  of  the  cell  surface  for 
certain  substances  and  the  character  of  the  substances  which  can 
react  with  the  enzyme  within  the  cell.  The  life  process  of  the 
organism  may  well  depend  upon  this  correlation.  The  study  of 
the  properties  of  cell  surfaces  or  membranes  would  therefore  be 
an  essential  part  of  the  study  of  enzyme  actions  within  the  cells 
for  a  proper  understanding  of  the  .chemical  phenomena  of  life 
processes. 


INDEX 


Authors'  Names  in  Roman,  Subjects  in  Italics 


Abderhalden,    E,    101,    112 
Abel,  E,  62 

Accelerated   reactions    (auto-cataly- 
sis), 28 

"Acclimatization"  135,  138 
Aceto-acetic  ester  decomposition,  50 
Aceto-acetic  ester  formation,  28 
Acree,  S.  F.,  107 
"Active"  molecules,  72,  84,  86 
Adam,  N.  K.,  155 
Adams,  E.  P.,  91 
Adaptation,  130,  135,  138 
Addition   compound   theory,  10,  38- 

43,  45,  49,  51,  52,   105,   110,   123, 

124,  127,  139,  152,  156,  162 
Adkins,  H.,  159 
Adsorption,  61,   120,    142,   143,   144, 

145,   147,   148,   149,   150,   151,  152, 

153,  154,  155,  162,  164 
Akerlof,  G.,  110 
Alpha  particles,  80 
Ammonia    synthetic    processes,    22, 

160,  161 
Ammonium  chloride  formation  and 

decomposition,  40,  45,  46,  53,  60 
Amylase,  97,  103,  111,  113,  114,  116, 

120,  136 

Amyloclastic  action,  104 
Armstrong,  E.  F.,  116,  156 
Arnheim,  F.,  101 
Arrhenius,  S.  A.,  16,  72,  83,  84,  86, 

88,  89,  116,  117,  118 
Asarnoj,  S.,  135 
Atom  of  electricity,  74 
Atoms,  structures  of,  73,  74,  75,  76, 

77,  78,  79,  80,  81,  82,  83,  92,  93, 

124,  149 
Avery,  O.  T.,  100 

Bacterial  enzymes,  134,  135,  136-138 

Bacterial  metabolism,  136-138 

Bacterial  urease,  134,  135 

Bahlke,   W.   H.,   161 

Baldwin,    M.   E.,    104 

Baly,  E.  C.  C.,  83,  89,  90,  150 

Bancroft,  W.  D.,  62,  155,  158 


Barker,  W.  F.,  89 

Baume,  G.,  105 

Bayliss,  W.  M.,  120,  162,  164 

Beans,  H.  T.,  126 

Benzene-chlorine  reactions,  51 

Berzelius,  J.,  11,  12,  13,  14,  15,  17, 

18,  19,  23,  52,  95 
Bien,  Z.,  105 
Bigelow,  S.  L.,  30 
Bimolecular  reaction  rate,  85,  87 
Bjerrum,  N.,  110 
Blood,  A.  F.,  Ill 
Blount,  E.,  104 
Bodenstein,  M.,  151 
Bohr,  N.,  78,  93 
Bohr  atom,  78,  93 
Beltzmann,  L.,  91 
Bern,  S.,  97 
Bradley,  H.  C.,  116 
Branch,  G.  E.  K.,  41 
Bray,  W.  C.,  112,  161 
Bredig,  G.,  13,  17,  23,  30,  45,  61,  62, 

125 

Brown,  A.,  115 
Brown,  H.  T.,  116 
Burns,  R.  M.,  155 
Bury,  C.  R.,  78 

Campbell,  G.  F.,  120 

Carbon    monoxide    oxidation,    112, 

153,   154,   161 
Catalase,  125 

Cells,  130,  131,  136,  163,  164,  165 
Chad  wick,  J.,  80 
Chemical  affinity,  12,  17,  41,  65,  66, 

67 

Clark,  R.  H.,  110 
Classification    of    catalytic    actions, 

15,  20 

Claude,  G,  22 
Co-enzymes,  111 

Colloidal,  properties  of  enzyme  prep- 
arations, 97,  98,  119,  162 
Colloidal  solutions   of   metals,   125, 

126 


167 


168 


INDEX 


Concentration  action  law,  25,  36,  37 

57,  61,   119 
Conover,  C.,  156 
Crafts,  J.  M.,  53 
Contact  catalytic  reactions,  60,  120, 

131,  140-165 
Contact  reactions,  140,  141,  151,  152, 

162,  163,  164,  165 
Continuity    of    life    processes,    127, 

128,  129,  130,  131,  132,  139 
Criteria  of  catalysis,  18,  19,  20,  23, 

26,  44,  52,  53,  56,  58,  63 
"Critical  increment/'  84,  85,  86 
"Cubical"  atom,  76,  78.  79 
Cullen,  G.  E.,  100,  110 
Guy,  E.  J.,  78 

Daniels,  F.,  88,  89 

Davidsohn,  H.,  99,  101,  105 

Davy,  E.,  11 

Deacon  chlorine  process,  22 

Definitions  of  catalysis,  11,  12,  13, 
16,  17,  21,  23,  27,  29,  30,  31,  32, 
36,  44,  53,  55,  56,  58,  61,  62,  63  64, 
65,  69,  70,  95,  124,  125,  140,  141 

Derick,  C.  G.,  66,  67 

Dernby,  K.  G.,  102 

Different  products  formed  by  cata- 
lysis, 12,  45,  46,  47,  49,  50,  51,  52, 
64,  94,  95,  96,  124,  125,  127,  157, 
158,  159 

Diffusion  rates,  27,  151 

Dobereiner,  J.  W.,  11 

Dual  theory  of  catalysis,  64,  107,  110 

Duclaux,  E.,  115 

Dulong,  P.  L.,  12 

Dushman,  S.,  87 

Eastlack,  H.  E.,  126 

Eastman,  E.  D.,  78 

Ebert,  L.,  110 

Einstein,  A.,  85,  86,  89,  90,  93 

Electrolytic  conduction  in  solutions, 

74 
Electrolytic  dissociation  theories,  42, 

43,  110,  122 
Electron  conception  of  valence,  42, 

43,  74,  75,  76,  80,  81,  149,  150 
Electron  emission,  144 
Electrons,  73,74,  75,  76,  77,  78,  79,  80. 

81,  82,  83,  91,  92,  93,  124,  144,  149, 

154 

Emulsin  20,  51,  113,  114,  116,  118 
Engelder,  C.  J.,  158 
Environment  action,  51,  129, 130, 134 
Enzyme  actions,  15,  20,  22,  51,  94- 

122,    125-131,    132,    133,    134,    136, 

137,  138,  161,  162,  163,  165 


Enzyme  actions  and  hydrogen  ion 
concentrations,  98,  99,  100,  101, 
102,  103,  104,  105,  106,  108,  111, 
113,  120 

Equations,  chemical  and  mathemat- 
ical, 25,  26,  27,  36,  58,  59,  60,  114, 
115,  119,  120 

Equilibria  in  chemical  reactions,  19, 
20,  21,  41,  46,  56,  57,  58,  61,  62,  63, 
64,  68,  118,  120,  157 
Equilibrium  constants,  57,  58,  61,  62, 

84 

Erepsin,  101,  102,  109,  112 
Esterase,  105,  112,  114,  116,  161 
Ester  hydrolysis,  16,  22,  29,  34,  35, 
38,  39,  40,  53,  54,  59,  64,  96,  99, 
104,   105,   106,   110,   113,   116,   121, 
122 

Esterification  38,  39,  40 
Ether  from  alcohol,  12,  22,  50 
Ethyl  acetate  decomposition,  159 
Ethyl  alcohol  decomposition,  12,  22, 

50,  157 

Ethyl  alcohol  oxidation,  11 
Ethylene  from  alcohol,  22,  50,  157 
Euler,  H.,  97,  100,  116,  134,  135,  136, 

157 

Evans,  C,  L.,  116 
Evolution,  130,  139 

Fales,  H.  A.,  99 

Falk,  K.  G.,  10,  22,  24,  38,  40,  41, 

42,  43,  64,  74,  76,  97,  99,  104,  105 

107,  111,  114,  116,  120,  150 
Faraday,  M.,  74 
Fermentation,  135,  137,  138 
Filament  and  gas  actions,  153 
Findlay,  A.,  164 
Fink,  C.  G.,  151 
Fodor,  A.,  101 
Forced  movements,  133 
Formic  acid  decomposition,  41,  48, 

158,  159 

Fortner,  M.,  125 
Frankel,  E.  M.,  102,  111 
Frazer,  J.  'C.  W.,  112,  161 
Free  energy  change,  16,  41,  46,  65,  66> 

67,  68,  72 
Friedel,  C.,  53 
Friedel-Crafts'  reaction,  53 
Fruit  fly  (Drosophila) ,  132 
Fry,  H.  S.,  76 

Gelatine  hydrolysis,  100,  101 
Ghosh,  J.  C.,  110 
Gibbs,  H.  D.,  156 
Glendinning,  T.  A.,  116 
Glimm,  E.,  114 


INDEX 


169 


Goldschmidt,  H.,  35,  107 

Goodrich,  E.  S.,  130 

Griffin,  E.  G.,  120 

Grignard,  V.,  39 

Grignard  reaction,  39 

Gross,  P.  M.,  43 

Growth,  128,  129,  130,  131,  137,  138 

Gurvich,  L.  G.,  147 

Haber,  F,  22,  160 

Hamlin,  M.  L.,  Ill 

Handovsky,  H.  112 

Harden,  A.,  135 

Hardy,  W.  B.,  148 

Harkins,  W.  D.,  142,  148,  150,  164 

Hedelius,  A.  Hj.,  157 

Heilbron,  I.  M.,  89 

Helmholtz,  H.  von,  74 

Heterogeneous  systems,  20,  27,  140, 

141,  151,  162,  163 
Hilditch,  T.  P.,  156 
Hitchcock,  D.  I.,  120 
Hofmann,  K.  A.,  158 
Hormones,  126,  127 
Howell,  W.  H.,  137 
Hoyer,  E,  111 
Hudson   C.  S.,  51 
Hull,  M.,  101,  109 
Hydrazine  decomposition,  51 
Hydrogenation    reactions,    22,    155, 

161 

Hydrogen-chlorine  reaction,  90 
Hydrogen  ion  actions,  15,  16,  59,  64, 

68,  98,  99,  100,  101,  102,  103,  104, 

105,  106,  107,  108,  109,  110,  111, 
113,  114,  115,  120,  121,  122 

Hydrogen-oxygen   reaction,    11,   12, 

53,  153,  154,  155 
Hydrogen   peroxide   decomposition, 

11,  30,  125 
Hydrolytic  reactions,  11,  16,  17,  20, 

28,  29,  32-36,  39,  53,  54,  59,  60,  64, 

96,  99,  100,  101,  102,  103   104,  105, 

106,  107,   109,  112,   113,   114,  115, 
116,  117,  118,  119,  120,  121,  122 

Hydroxylamine  decomposition,  159, 

160 
Hydroxyl  ion  actions,  15,  17,  54,  66, 

67,  99,  100,  101,  103,  104,  105,  106, 

108,  113,  114,  121,  122 


Ikeda,   K.,   125 

Imido  ester  reactions,  32-36,  48,  54, 

65,  66,  67,  68,  69,  105,  109,  122 
Inactivation  of  enzymes,  97,  98,  99, 

106,  107,  108,   114,  120,  125,  126, 

127,  128,  129 


Industrial  applications  oj  catalysis, 

22 

Infra-red  radiation,  85,  86 
"Inorganic  ferments/'  125,  126 
lonization  of  enzyme  preparations, 

108,  109 

Jacoby,  M.,  134 
Jeans,  J.  H.,  93 
Jennings,  J.  M.,  161 
Johnson,  J.  M.,  107 
Johnston,  E.  M.,  88,  89 

Kallman,  H.,  110 

Kay,  S.  A.,  20 

Kelber,  C.,  155 

Kendall,  A.  I.,  136,  137,  138 

Kendall,  J.,  43,  105 

Kephirlactase,  118 

Kinetic  theory,  71 

King,  H.  H.,  150 

King,  H.  S.,  78 

Kirchhof,  J.,  11 

Kossel,  W.,  76 

Krause,  A.  C.,  159 

Kruyt,  H.  R.,  148 

Kubota,  B.,  158 

Lactase,  116 

Lamb,  A.  B.,  112,  161 

Langmuir,  I.,  76,  77,  78,  79,  88,  89, 
93,  142,  143,  144,  145,  146,  147,  148, 
149,  150,  151,  152,  153,  154,  158, 
159,  162,  163,  164 

Latimer,  W.  M.,  78 

Laurin,  I.,  135 

"Law  oj  survival  oj  the  unattrac- 
tive," 78 

Lehfeldt,  R.  A.,  20,  71 

Lewis,  G.  N.,  76,  77,  79 

Lewis,  W.  C.  McC.,  71,  84,  85,  86, 
87,  88,  90,  107 

Lind,  S.  C.,  18 

Lipase,  97,  105,  109,  111,  112,  113, 

116,  118,  120 
Liquid  surfaces,  146,  148 

Living  matter,  94,  95,  96,  123,  125, 
127,  128,  129,  130,  131,  132,  139, 
163,  165 

Loeb,  J.,  131,  132,  133,  163 

Long,  J.  H.,  101,  109 

Maclnnes,  D.,  110 

Magneton  theory,  77 

Mailhe,  A.,  50,  157 

Maltose,  109,  112,  116,  118 

Marcelin,  R.,  84,  86,  91 

Mass  action  law,  24,  25,  39,  48,  57, 

117,  119,  133,  152 


170 


INDEX 


McGuire,  G.,  99,  104,  114 

McKeown,  A.,  90 

Mechanism  of  catalytic  actions,  29, 
30,  31,  32-36,  45-47,  48,  51,  52,  54, 
55,  69,  118,  119,  156,  158,  160,  163 

Mechanism  of  chemical  reactions, 
10,  21,  25,  38-43,  44,  48,  54,  55,  56, 
68,  70,  92,  94,  105,  106,  119,  150, 
151,  152,  155,  157,  160,  163 

Membranes,    131,    163,    164,    165 

Mendel,  L.  B,  111 

Mendeleeff,  D.,  79 

Mendelssohn,  A.,  101 

Menten,  M.  L.,  113,  115 

Menzies,  A.  W.  C.,  78 

Merrill,  D.,  161 

Micelles,  131 

Michaelis,  L.,  99,  101,  109,  113,  115. 

Miller,  E.  W.,  134 

Millikan,  R.  A.,  93 

Milner,  S.  R.,  110 

Mitcherlich,  E.,  12 

Molecules,   electronic  structures  of, 

70,  73,  74,  75,  76,  77,  78,  79,  80, 
81,  82,  83,  92,  93,  124,  149 

M onomolecular    reaction    rate,    27, 

71,  72,  73,82,86,87,91,92,  101,  124, 
151 

Muller,  J.  A,  49 

Miiller  von  Berneck,  R.,  125 


Negative  catalysis,  30,  31,  32,  48 
Nelson,  J.  M,  40,  42,  43,  74,  97,  99, 

105,  107,  113,  115,  120,  150,  163 
Nernst,  W.,  151 
Neun,  D.,  101 
Neutral  salt   actions,   33,   107,   110, 

111,    161 

Neville,  H.  A.,  155 
Nicholson,  J.  W.,  93 
Norris,   R.  V.,   104,   135 
Northrop,  J.  H.,  100,  102,  103,  109, 

113,  114,   117,   118,   119,   120,   121, 

132,  133 

Noyes,  A.  A.,  110,  149,  151 
Nucleins,  131,  132 
Okada,  S.,  101,  102 

Orientation  of  molecules  on  surfaces, 

144,  146,  147,  148,  154 
Osborne,  T.  B.,  97,  120 
Osmosis,  163 
Osmotic  pressure,  164 
Ostwald,  W.,  13,  16,  17,  18,  19,  20, 

21,  23,  27,  29,  30,  45 
O'Sullivan,  C.,  120 


Oxidation  reactions,  15,  22,  29,  30, 
112 

Oxidation-reduction  actions,  11,  13, 
14,  15,  22,  29,  30,  41,  48,  50,  51,  53, 
96,  111,  119,  125,  153,  157,  161 

Oxynitrilase  118 

Pamfil,  G.-P,  105 

Papain,  102,  111 

Parson,  A.  L.,  77 

Partington,  J.  R.,  78 

"Passive"  molecules,  72,  84,  86 

Pease,  R.  N.,  78,  161 

Peirce,  G.,  116 

Pekelharing,  C.  A.,  109 

Pepsin,  101,  102,  103,  109,  112,  113, 

114,  116,  117,  118,  120,  121 
Periodic  System,  78,  79,  80 
Perrin,  J.,  86,  88 
Pettersson,  A.,  135 
Peytral,  E.,  49 
Photochemical  equivalence  law,  85, 

86,  89,  90,  93 
Photochemical  reactions,  18,  88,  89, 

90,  91,  93 

Physical    properties    of    substances, 

9,  42,  79,  82 
Piggot,  C.  S.,  161 
Planck,  M.,  85,  91 
Planck's  constant,  85,  91 
Porter,  A.  W.,  30 
Positive   nuclei,   73,  74,  77,   78,   79, 

80,  81,  82,  83 
Preferential    combustion    reactions, 

155,  156 
Primary  valences,  142,  143,  149,  150, 

154 

Probability  law,  82,  89,  93 
Production    of    enzymes,    128,    129, 

130,  132,   134,   135,   136,   137,   138, 

139 

Promoter  action  in  catalysis,  161 
Protease,  101,  102,  103,  109,  112,  113, 

137 
Protein  hydrolysis,  95,  96,  99,  100, 

101,   102,   103,   104,   106,   112,   113, 

116,  117,   118,   127 
Proton,  80 
Purity  of  enzyme  preparations,  97, 

98,  109 

Quantum  theory,  85,  87,  88,  89,  90, 

91,  92 

Radiation,  70,  82,  83,  85,  86,  87,  88, 

89,  90,  91,  92,  93,  125,  140,  141 
Radioactive  substances,  73 
Rankine,  A.  O.,  78 


INDEX 


171 


Reaction  velocity,  16,  17,  18,  19,  20, 
23-37,  39,  41,  45,  46,  47,  52,  53,  54, 
56  57,  58,  62,  63,  64,  65,  68,  71, 
72^  73,  79,  82,  83,  86,  87,  88,  89, 
96,  99,  100,  101,  105,  106,  110,  111, 
112,  113,  114,  115,  116,  117,  118, 
119,  124,  125,  126,  127,  132,  151, 
152 

Reaction  velocity  constants,  27,  28, 
31,  32,  36,  39,  56,  57,  58,  63,  83, 
86,  88,  99 

Rearrangements,  non-reversible,  66, 
67 

Refractive  index,  86,  91 

Regeneration,  132,  133 

Reinders,  W.,  125 

Resistance,  chemical,  72,  73 

Reversible  reactions,  56,  57,  58,  63, 
118 

Reynolds,  W.  C.,  148 

Rice,  F.  O,  83,  84,  86 

Rideal,  E.  K,  19,  62,  156,  161 

Ringer,  W.  E.,  109,  120 

Rodebush,  W.  H.,  78 

Rogers,  T.  H.,  161 

Rona,  P.,  101,  105 

Rosanoff,  M.  A.,  62 

Rutherford,  E.,  80 

Sabatier,  P.,  50,  156,  157,  158,  159 
Saccharogenic  action,  104 
Scalione,  S.  C.,  161 
Schibsted,  H.,  158 
Schlesinger,  M.  D.,  97,  104 
Schiitz,  E,  115,  116,  117,  118 
Schutz's  rule,  115,  116,  117,  118 
Secondary    valences,    142,    143,    145, 

149,  150 
Semi-permeable     membranes,     161, 

163,  164,  165 
Senter,  G.  A.,  30 
Sherman,  H.  C.,  97,  101,  104,  120 
Silica  gel  actions,  160 
Simultaneous  reactions,  34,  48,  59, 

68,  127 
Slator,  A.,  51 
Solvent  action,  20,  21,  26,  37,  42,  43, 

59,  61,  62,  107 
Sorensen,  S.  P.  L.,  99,  101 
Specificities    of   chemical   reactions, 

112,  113,  155,  156 
Specificities  of  enzyme  actions,  112, 

113 
Spectra  of  elements  and  compounds, 

78,  79,  88 
Starch  hydrolysis,  11,  96,  103,   104, 

116 
Starling,  E.  H.,  120 


Statistical    mechanics    applications, 

57,  84,  87,  89,  92 
"Steric"  factor  in  reactions,  71,  81, 

147 

Stern,  O.,  81 
Stieglitz,  J.,  23,  30,  32,  35,  36,  45, 

48,  54,  62,  65,  66,  67,  68,  105,  107, 

109,  122 

Stohmann,  F.,  16 
Substrate  configuration,  109,  121 
Successive  reactions,  26,  29,  33,  35, 

36,  53,  54,  60,  103,  104,   106,  115, 

116,  119,  125,  132,  163. 
Sugiura,  K.,  97,  116 
Sutherland,  W.,  110 
Sucrose,   51,   97,   99,    100,    109,   112, 

113,  114,  115,  120,  134,  161 
Sucrose   hydrolysis,   16,    17,   53,   64, 

96,  99,  110,  112,  115,  121 
Sulfuric  acid  production,  22,  160 
Surface  action  law,  152 
Surface   forces,    142,    143,    144,    145, 

146,  147,  148,  149,  150 
Surface  properties,  142,  143,  144,  145, 

146,   147,  148,   149,   152,   154,   156, 

160,  162,  163,  164,  165 
Surface  tensions,  146,  148 
"Survival  of  the  fittest,"  130 
Svanberg,  O.,  97,  150 
Synthetic  actions  of  enzymes,  118, 

128,  132 

Tanaka,  Y.,  Ill 

Tanatar,  S.,  51,  159 

Taylor,  H.  S.,  19,  62,  155,  156,  161 

Temperature   and  duration  of  life, 

132 
Temperature  and  reaction  velocity, 

24,  27,  71,  72,  73,  83,  84,  85,  86, 

94,  95,  108,  125,  132,  153 
Thenard  J.,  11,  12 
Thermodynamic   considerations,   19, 

57,  58,  67,  92,  147 
Thiele,  J.,  150 
Thomas,  A.  W.,  104,  111 
Thomson,  J.  J.,  74,  78,  79 
Tolman,  R.  C.,  87,  88,  89 
Tompson,  F.  W.,  120 
Trautz,  M.,  81,  86,  87,  88 
"Trigger"  action,  89,  93 
Tropisms,  133 
Trypsin,  101,  102,  103,  109,  112,  113, 

118 
Twitchell,  E.,  22 

Udby,  O.,  35 

Urea  decomposition,  96,  112,  116 

Urease,  112,  116,  134 


172 


INDEX 


Valence,  42,  74,  75,  77,  78,  79,  80,  82,      Waldschmidt-Leitz,  E.,  155 


142,  143,  145,  149,  154,  159 
Van  Duin  C.  F.,  148 
Van  Slyke,  D.  D.,  116 
Van't  Hoff,  J.  H.,  13,  20,  21,  57,  58, 

71,  72,  84 
Vines,  H.  S.,  Ill 
Viscosity,  71 
Vitamins,   126,   127 
Vosburgh,  W.  C.,  115,  163 


Walker,  J.,  20 
Whitney,  W.  R.,  151 
Wien,  W.,  89 
Wijs,  J.  J.  A.,  104 
Wilhelmy,  L.,  17 
Willstatter,  R.,  155 
Wohl,  A.,  114 
Woog,  P.,  155 

Yeast,  134,  135 


This 


YC  21383 


THE  UNIVERSITY  OF  CALIFORNIA  LIBRARY 


